Diploid gynogenesis was induced in Japanese crucian carp (Carassius cuvieri) eggs using UV-irradiated genetically inactive spermatozoa from mirror carp (Cyprinus carpio L.) or blunt snout bream (Megalobrama amblycephala), with or without cold shock. The optimal radiation dosage was 4,200 mJ/cm2 and 3,600 mJ/cm2 for mirror carp and blunt snout bream sperm, respectively. At this dosage and without cold shock, the yields were (32.4 +/- 3.3)% vs. (33.8 +/- 1.4)% gynogenetic haploids and (0.7 +/- 0.3)% vs. (0.5 +/- 0.3)% hybrid diploids, respectively. At the optimal UV dosage but with cold shock (2 min after fertilization, 0-4 degrees C for 40 min), the hatching rates were (27.8 +/- 2.1)% and (29.4 +/- 3.3)%, respectively. From hatching to feeding, (15.7 +/- 3.4)% and (23.6 +/- 4.1)% normal gynogenetic diploids were recorded, respectively. Survival of normal gynogenetic diploids was 56% out of the hatched fry when using irradiated spermatozoa of mirror carp, which was lower than that (up to 80%) when using irradiated spermatozoa of blunt snout bream. This indicated that the sperm of blunt snout bream, with distant genetic relation to the maternal Japanese crucian carp, was more effective than that of mirror carp to induce diploid gynogenesis. The nature of the gynogenetic progeny was identified with external appearance, chromosome number and gonad structure. The presence of only females in gynogenetic progeny probably suggested XX genotype in the female Japanese crucian carp. The gynogenetic diploids have potential values such as faster growth and stronger disease resistance than the normal Japanese crucian carp. All gynogenetic progeny possessed 100 chromosomes whereas all J x B crosses were triploid with 124 chromosomes. The formation of the new triploid hybrids in J x B crosses may be useful in aquaculture.