Larvae of Tetragoneuria cynosura were collected in 1965—1966 and 1969 to determine their responses to photoperiod and temperature and to elucidate the mechanisms by which seasonal regulation is achieved. Animals were subjected to experimental conditions of temperature (15°, 20°, 25°, and 30°C) and photoperiod (11, 14 hr) until emergence. Three additional groups in each collection were subjected to 11—hr, 14—hr, and natural photoperiods at lake temperatures. In larvae collected after the autumnal equinox, development progressed rapidly in animals subjected to the 14—hr day at every experimental temperature. Short—day animals experienced delayed developed especially at 15° and 30°C. Animals maintained at natural temperatures responded similarly regardless of their photoperiod regime. Short days and high temperatures induced delayed development in all animals collected in the fall and winter. For larvae collected prior to September, long days greatly retarded growth compared to short—day animals. In larvae collected from mid—September until mid—October, the rate of response to long days increases dramatically. This reversal in responseoccurred uniformly about 12 days following the autumnal equinox at 20°, 25° and 30°C. A strong correlation exists between this shift in response to experimental long days and entry into the final instar. The presence of a diapausing stage could not be fully ascertained although development was significantly delayed by experimental long days in animals collected prior to the autumnal equinox and by short days in animals collected following the equinox.