Single-cell protein (SCP) refers to the dried cells of microorganisms. The aim of this research was to evaluate the nutrional characteristics and possible toxic effects of the SCP of Trichoderma harzianum. First, T. harzianum was grown on whey filtrate agar medium and the obtained SCP was analysed. It was rich in both total protein (34.21%) and ash (4.78%). Furthermore, the biomass contained all the essential amino acids, and the amino acid concentrations were very close to the FAO reference protein levels. Second, we exposed zebrafish (Danio rerio) embryos to diluted SCP at various concentrations for 96 hours postfertilization (hpf). Compared with the control group, we did not observe any developmental abnormalities, delayed hatching, and lethal effects on zebrafish embryos (96 hpf) found in the SCP group. To test diet effects on spawning success and growth of embryos, adult zebrafish were fed on SCP and flake feed diets for 10 weeks. The number of laid eggs, wet weight and diameter of eggs, and the percentages of hatched eggs from fish fed the flake diet and SCP diet were not significantly different from each other. Also, larval length and weight were not significantly affected by diets. Finally, SCP did not cause any toxic effect on zebrafish adults and their offsprings and could be useful as fish food or food additive.