Eggs of carp (Cyprinus carpio) were inseminated with spermatozoa genetically inactivated by ultraviolet radiation, and were incubated at 20°C. In the 1st of 2 experiments, batches of 200-500 eggs were heat-shocked at 37°, 39° or 41° for 3 min, starting between 2.8 and 58 min after insemination. In the 2nd experiment, the same treatments were used, but heat shock was applied between 2.8 and 86.8 min after insemination. Heat shock at 37° did not produce diploid larvae. After heat shock at 39°, starting 2.8 min after insemination (to prevent the 2nd meiotic division), the percentage of hatched eggs that produced diploid larvae of normal appearance was 55.3±5.1 and 72.6±3.3 in the 2 experiments. Corresponding values for eggs treated at 41° were 32.1±4.5 and 16.5±3.5%; values for eggs treated at 41° starting 53.2 and 58.8 min after insemination (to prevent the 1st cleavage division) were 27.8±4.0 and 50.0±25.0% resp.