Abstract
Although the Sertoli cell tumour is a common testicular neoplasm in the dog, accounting for 32% of testis tumours in one study (Nielsen & Lein 1974), we have only found one report (von Bomhard et al. 1978) on the ultrastructure of this tumour in the dog. The present paper, describing electron microscopic features of one such tumour in an undescended testis of an 11-year-old dog, provides some new information on the ultrastructure of the neoplasm.