Judgment unanimously reversed on the law without costs and new trial granted. Memorandum: Supreme Court erred in precluding defendant doctor from offering expert testimony that the cause of the death of plaintiff's infant was umbilical cord stricture. The court precluded that testimony because defendants failed to identify defendant doctor as an expert and disclose the subject on which he was expected to testify, as required by CPLR 3101 (d) (1) (i). Because defendant doctor was plaintiff's treating physician, rather than an expert retained to give opinion testimony at trial, CPLR 3101 (d) (1) (i) does not bar the admission of his expert testimony ( see, Rook v. 60 Key Centre, 239 A.D.2d 926; Beck v. Albany Med. Ctr. Hosp., 191 A.D.2d 854, 856; Nesselbush v. Lockport Energy Assocs., 169 Misc.2d 742). Defendants were significantly prejudiced by the error because they were thereby prevented from presenting a defense. Moreover, the error was compounded when the court permitted plaintiff's expert to testify that the infant's death was not caused by umbilical cord stricture. Consequently, we reverse the judgment and grant a new trial.