The presence of trehalase and trehalose phosphorylase in L3 and L4 larvae of Anisakis simplex was demonstrated. The activity of trehalase and trehalose phosphorylase in L3 larvae was 6 and 10 times higher, respectively, than in L4 larvae. This suggests that trehalose metabolism is more important for L3 than L4 larvae. Trehalases of L3 and L4 differ in their characteristics. The enzyme of L3 was present mainly in the lysosomes and cytosol, whereas in L4 the highest enzyme activity was measured in the lysosomal fraction. Trehalase activity was increased by 29% in L3 and 55% in L4 with the addition of Mg² (0.1 mmol). Tris inhibited trehalase in L3 larvae by 42% and in L4 by 25%. The enzymes differed in their reaction to EDTA, CaCl₂, ZnCl₂, and CH₂ICOOH (all 0.1 mmol). High activity of trehalase from L₃ larvae was measured within the pH range of 5.0 to 6.5, with an optimum pH of 6.1. The trehalase was a thermally tolerant enzyme from 25 C to 60 C. The enzyme lost half of its activity after preincubation without substrate above 75 C. The paper also discusses the similarities and differences in characteristics of trehalase from A. simplex larvae and presents the comparison to enzymes from other nematodes.