Habitat alterations and physical adaptations putatively advantageous to northern pike Esox lucius have been proposed as probable causes for the relative population size changes of northern pike and muskellunge E. masquinongy. Area estimates were made of historical and recent spawning habitat to test the hypotheses that spawning habitat and location of spawning have changed over time. Development and growth of eggs and larvae under varying regimes of temperature and diet were examined to test the hypotheses that northern pike larvae grow faster than muskellunge larvae, that adhesive papillae are present only in northern pike larvae, and that northern pike eggs and larvae are more tolerant of colder water than are muskellunge eggs and larvae. The effect of changes in spawning habitat on relative time of occurrence of Esox, zooplankton, and forage fish was examined in three embayments of the upper St. Lawrence River. Estimated hectares of shallow emergent vegetation declined by an average of 84%, and hectares of Typha increased by an average of 46% from 1948 to 1995. Eggs of both species were adhesive until water hardening. Larvae of both species possessed adhesive papillae, but muskellunge used their papillae for less time than did northern pike. Northern pike larvae attained various developmental characters at an earlier age than did muskellunge larvae in controlled and varying water temperatures. Growth of most body structures was similar but snout length was greater in muskellunge larvae. Northern pike larvae grew faster than muskellunge larvae in length and weight on diets of Artemia and fish larvae. Both species grew faster on fish larvae and could consume fish larvae as a first food. No transition from zooplankton was necessary. Cladocerans dominated the zooplankton with Polyphemus and Diaphanasoma being more abundant than other taxa. Yellow perch Perca flavescens dominated the fish larvae accounting for 93% of the total. Overall density of fish larvae was 10 per cubic meter. Copepods were the primary prey taxa of fish larvae. Northern pike and muskellunge larvae (collected as eggs) were estimated to be capable of feeding on fish larvae during or near to the end of the peak forage fish density in one bay while both species were at this condition one week prior to the peak forage fish density in another bay.