Ultrasonography was used for diagnosing accumulation of rete testis fluid in live bulls. When 447 bulls of Ayrshire breed selected for artificial insemination were studied by ultrasonography, eleven were detected in which the ultrasonograms visualized accumulation of rete testis fluid in the testicles. In 7 bulls the accumulation (spermiostasis) was bilateral and in 4 unilateral. In only four out of 18 testicles, macroscopically epididymal cysts were seen after slaughter. At the age of 16 months four bulls with bilateral spermiostasis were azoospermic. When studied by ultrasonography, the rete testis of normal testicles was echogenic and narrow but in affected testicles anechogenic and wide. Nine of the affected bulls were sons of three fathers. We suggest that spermiostasis is hereditary in Ayrshire bulls and, as in the goat, is progressive by nature.