
704 S.W.2d 911 (1986)
Thomas F. BREWER and Texas Utilities Fuel Company, Appellants,
v.
Rodney P. ISOM and Wife, Filena D. Isom, Appellees.
No. 05-85-00574-CV.
Court of Appeals of Texas, Dallas.
January 28, 1986.
Ronald H. Clark, Henderson, Bryant & Wolfe, Sherman, for appellants.
Smith E. Gilley, Greenville, for appellees.
Before STEPHENS, McCLUNG and McCRAW, JJ.
STEPHENS, Justice.
Thomas F. Brewer and Texas Utilities Fuel Company (appellants) appeal from an adverse judgment in favor of Rodney P. Isom and Finela D. Isom (appellees). In their sole point of error, appellants contend the trial court erred in admitting the testimony of a witness whose name had not been disclosed to appellants in answers to interrogatories or in depositions. We agree. The judgment is reversed and the cause is remanded for a new trial.
At trial, appellees sought to introduce testimony of their neighbor, W.W. Oliver. Appellants objected on the grounds that appellees had not disclosed the name of the witness in response to interrogatories or in depositions as a person with knowledge of facts and circumstances pertaining to the lawsuit. Appellant claimed unfair surprise and the lack of an opportunity to depose the witness, investigate his testimony, or obtain rebuttal witnesses. Appellees' attorney stated that the witness's testimony would only be cumulative and that there would be no other such witnesses. After the court overruled appellants' objection, appellants moved for a continuance claiming surprise. This motion was denied and the trial continued. The jury returned a verdict for appellees finding appellants negligent and grossly negligent, and the trial court rendered judgment based on that verdict.
Rule 166b(5) of the Texas Rules of Civil Procedure states in pertinent part:
A party who has responded to a request for discovery that was correct and complete when made is under no duty to *912 supplement his response to include information thereafter acquired, except the following shall be supplemented not less than thirty days prior to the beginning of trial unless the court finds that a good cause exists for permitting or requiring later supplementation.
a. A party is under a duty seasonably to supplement his response if he obtains information upon the basis of which:
(1) he knows that the response was incorrect or incomplete when made;
(2) he knows that the response though correct and complete when made is no longer true and complete and the circumstances are such that failure to amend the answer is in substance misleading....
Rule 215(5) of the Texas Rules of Civil Procedure states:
A party who fails to supplement his response to a request for discovery in accordance with paragraph 5 of Rule 166b shall not be entitled to present evidence which the party was under a duty to provide in a supplemental response or to offer the testimony of an expert witness or of any other person having knowledge of discoverable matter when the information required by Rule 166b concerning the witness has not been disclosed, unless the trial court finds that good cause sufficient to require admission exists.
Appellants requested the names of all witnesses to a part or all of the events made the basis for the lawsuit. W.W. Oliver's name was not listed in appellees' response. When appellees' attorney located Oliver and learned he had knowledge of pertinent facts, appellees' attorney then knew that his initial answer was incomplete when made or no longer true. Rule 166b(5) required that appellees' attorney supplement his answer. This was not done. Therefore, a showing by appellees and a finding by the trial court that good cause existed for allowing Oliver to testify became the only basis for permitting his testimony. Yeldell v. Holiday Hills Retirement & Nursing Center, Inc., 29 Tex.Sup.Ct.J. 103, 105, 701 S.W.2d 243 (1985). Because appellees were required to make a showing of good cause and failed to do so, we conclude the trial court erred in allowing the testimony. We next determine whether reversal is required in this case.
Appellees alleged that appellants were negligent and grossly negligent in allowing oil to spill onto their land. The extent and number of spills were contested issues. Oliver, a downstream neighbor of appellees, testified about the spills and the effect they had on his property. In addition, he testified that the spills had killed the fish in his pond, and that his horses would no longer drink from the stream. He also testified about a previous unrelated spill. This non-cumulative evidence was from the only unpaid, non-interested witness in the case. In light of the fact that punitive damages were awarded by the jury, we conclude that the testimony was reasonably calculated to cause and probably did cause the rendition of an improper judgment in the case. See First Employees Insurance Co. v. Skinner, 646 S.W.2d 170, 172 (Tex.1983). Accordingly, the judgment is reversed and the cause remanded for a new trial.
