Forty-six bulls between 17 and 23 months old were used in an investigation of testicular hypoplasia in Bos indicus strain bulls. Testes diagnosed as either unilaterally or bilaterally hypoplastic or normal were obtained at castration and were examined macroscopically and by quantitative histological methods. Estimations of sperm per gram of testis (spg) were made on these 46 bulls and two others. Testis weight was greatest in the 'normal of unilateral' group (106 g). The mean epididymis:testis weight ratio was 0.12 and did not differ significantly between groups. Increased basement membrane thickness, increased vacuolation and giant cells were not features of testicular hypoplasia per se. Seminiferous tubule diameter varied from 118 micrometer in bilaterally hypoplastic testes to 171 micrometer in normal testes. A mean tubule diameter of about 150 micrometer was considered useful for differentiating hypoplastic from normal testes in young post-puberal B indicus crossbred bulls. There was a significant correlation between tubular diameter and spg (r = 0.68, P less than 0.001) suggesting that tubular diameter measurements in histological sections could be used to predict sperm production. Although there was good agreement between clinical diagnosis and spg values, high spg values were found in several small testes which were considered hypoplastic on clinical examination. Examination of both unilaterally and bilaterally hypoplastic testes showed an interesting variation in the ratio of nucleus to cytoplasm in the interstitial cells. This was significantly (p less than 0.05) lower in the testes of normal bulls than in the other clinical groups.