Laboratory experiments were conducted to determine the effects of methoxychlor on the larval development of the mud-crab, Rhithropanopeus harrisii, and the commercial blue crab, Callinectes sapidus, from the time of hatching until the 1st crab stage was reached. The effect of a range of concentrations of methoxychlor on survival of larvae of C. sapidus and R. harrisii was determined, as well as the concentrations which were sublethal and lethal. Since concentrations as low as 1.3, 1.6 and 1.9 ppb (μg l−1) methoxychlor were acutely toxic to C. sapidus larvae, and it took a concentration as high as 7.0 ppb to be acutely toxic to R. harrisii larvae, it was concluded that C. sapidus larvae were much more sensitive to methoxychlor than R. harrisii larvae. Zoeal and total development to the 1st crab stage of R. harrisii and C. sapidus were prolonged in relation to increased concentrations of methoxychlor. The developmental stages in which larvae were particularly sensitive varied in the two species. Methoxychlor residues of R. harrisii and C. sapidus larvae reared in concentrations of methoxychlor were determined.