The aim of our study was to investigate acute and chronic effects of the oil cleaner Simple Green (SG), crude oil (oil), crude oil and SG mixture (oil-SG) on rainbow trout (Oncorhynchus mykiss) in early development stages (embryos, larvae). The acute effects of SG, oil, and oil-SG mixture on rainbow trout embryos and larvae were investigated. The 96-hour LC₅₀ values for embryos/larvae were: SG – 1,270/1,040; oil – 39,280/21,610; and oil-SG – 24,430/14,040 mg/l. Larvae were more sensitive for these toxicants than embryos. The chronic effect of oil-SG mixture on rainbow trout in early stages of development showed that the mixture slightly increased the mortality of embryos, and influenced more the mortality of larvae. The mortality of larvae depends on the duration of exposure and oil concentration in the mixture. Comparative studies on the acute and chronic effects of oil and the oil-SG mixture on rainbow trout in early stages of development showed that the mixture has a more toxic effect on embryos and larvae than oil alone.