Sympatric populations of Ambystoma opacum and Ambystoma t. tigrinum were studied in ponds on the coastal plain of New Jersey. Ambystoma tigrinum bred during the "January thaw" and its eggs remained under ice for a month after deposition. Eggs were laid in the deepest parts of the ponds. The mean number of eggs per mass (44) was lower than reported elsewhere for A. t. tigrinum. Development of eggs was followed in the field. The cold temperatures extended the breeding season (60 days) and prolonged the incubation period (45-50 days). After hatching in March, the larvae metamorphosed in June (75 days) at 40.5 mm snout-vent (SV) length, 70.3 mm total length. The prolonged breeding, egg-laying and incubation periods, with a short larval period and metamorphosis at a small size are adaptive to breeding in temporary ponds in this region. Eggs of Ambystoma opacum hatched when the ponds filled in midwinter after an incubation period of 120 days. The larval period (135 days) was prolonged because of low environmental temperatures. Growth and proportional changes of the larvae were studied. Most of the growth occurred in the last 60 days of the larval period. Metamorphosis occurred at 38.0 mm SV, 67.7 mm total length, exceeding that attained by larvae in Maryland or North Carolina. Metamorphosis in A. tigrinum was triggered by extrinsic factors (pond drying), and that of A. opacum at least partly by intrinsic factors. Ontogenetic and diurnal changes in behavior and habitat selection were studied in both species. Ontogenetic behavioral changes were correlated with development of limbs and distinct species differences were observed. A. opacum exploited the open-water resources early in the year and inhabited the bottom or vegetation when tigrinum larvae utilized open water. Both species remained hidden in vegetation during the day and were active only at night when they fed. Specific differences in growth and development, and the successive occupation of different zones in the ponds permitted coexistence with little competition.