This paper covers the findings of 10 years of research on the problem of the low fertility of brown trout eggs (Salmo trutta) at the Jonkershoek Inland Fish Hatchery, Stellenbosch. As a background to the problem a brief résumé is given of the acclimatization of trout in South Africa with which the development of the Jonkershoek Hatchery has been closely associated.