Malachite green (MG) is frequently used to treat fish pathogens in zebrafish, but little is known about how MG may affect the physiological processes that are targets of research investigations. This study evaluated the effects of MG on reproductive success in zebrafish, Danio rerio, and the relative toxicity of MG oxalate and MG chloride salts. After 5-day exposure to a prophylactic concentration (0.065 mg/L) of MG oxalate, the percentage of spawning pairs significantly declined (57.5%+/- 8.5 SE) and then returned to pre-exposure levels (89%+/- 6.6 SE) within one week. Embryo and larval survival were not affected by exposure of parents to MG; however, when exposed directly, MG oxalate was more toxic than MG chloride in both life stages. The LC50 for embryos was 0.067 mu M and 0.116 mu M for MG oxalate and MG chloride, respectively, and the LC50 for larvae, 0.057 mu M and 0.103 mu M, respectively. Other than a transient reduction in the number of fish that spawned, MG treatment of broodstock did not impact reproductive success, which indicates that MG can be used prophylactically with minimal effects on reproduction. Based on acute toxicity, MG chloride should be preferred over MG oxalate.