We investigated the sensitivity of two freshwater crayfish species (Astacus astacus and Procambarus virginalis) during embryonic development to chronic exposure to the herbicide terbuthylazine under laboratory conditions. The assessed parameters included time of embryonic development, survival rate, hatching weight and histopathology of hepatopancreas. LC50 (median lethal concentration) and ED50 (median effective concentration) were estimated. We were able to determine effects of terbuthylazine for every investigated parameter. For noble crayfish, the LC50 value after 45 days was 0.11 mg/L, and the histology of the hepatopancreas showed effects starting from 0.025 mg/L. Other parameters revealed effects starting at concentrations of 1.6 mg/L for weight and 6.4 mg /L for embryonic development time and hatching rate. Marbled crayfish only showed effects concerning the hatching rate and survival rate at concentrations without a clear dose-effects curve. As a conclusion, our data shows the risk of terbuthylazine in existing concentrations in freshwater ecosystems to non-target organisms and also the need of toxicological studies on directly affected species in addition to the use of model organisms.