Gynogenesis is thought to be a useful method to generate fully inbred lines in teleost fish. Endomitosis, which refers to the inhibition of first mitosis, should lead to fully homozygous offspring. In this study, the optimal conditions to induce mitogynogenesis (endomitosis) by heat shocking of the eggs were determined in the African catfish Clarias gariepinus. Comparable survival rates were obtained with a shock of 40 degrees C applied for 1 min and a shock of 39 degrees C applied for 1.5-2 min. Applying the shock around 20 or 37 min after activation resulted in the highest survival rates (ca. 5% relative to the positive control). The amount of inbreeding (homozygosity) and paternal contamination were assayed by using polymorphic microsatellite DNA markers. In less than 1% of the offspring paternal alleles were present, indicating a true gynogenic background for most of the progeny. Meiogynogenetic and mitogynogenetic C. gariepinus showed a fair amount of residual heterozygosity (respectively 86% and 75% for one of the markers). Heterozygous catfish obtained through mitogynogenesis were attributed to the (simultaneous) presence of meiogynogens. In addition, we prove that the sex determining system of African catfish C, gariepinus is heterogametic in the male (XY/XX).