Phenology and states of dormancy ofUtricularia vulgariswere studied using plants and turions collected at intervals throughout the growing season of 1977 from a shallow eutrophic pond in central Alberta located at 53°25′ N, 113°46′ W. States of dormancy and the effects of photoperiod and temperature were determined by the potential for sprouting and subsequent apical growth under long-day (18 h, 20 °C), short-day (12 h, 20 °C), and high-temperature (16 h, 30 °C) conditions, with and without varying periods of chilling at 2–3 °C. Physiological changes associated with different states of dormancy such as frost hardiness, starch content, and dark respiration were also investigated. Turions were formed 1 month after summer solstice and entered a state of innate dormancy during which only a high-temperature treatment at 30 °C would induce sprouting. Plants from innately dormant turions that were forced to sprout and were then cultured at 20 °C invariably reentered dormancy by the production of new turions. Turions collected in the late stages of innate dormancy could withstand −8 °C but not −12 °C. As temperatures dropped and ice cover became continuous, turions entered an imposed state of dormancy during which immediate sprouting and continued vegetative growth occurred under all conditions. Starch content reached a maximum of 49 mg g−1fresh weight during early innate dormancy and declined steadily thereafter. Dark respiration in response to temperature of plants obtained from turions collected during innate and imposed dormancy states differed above 25 °C but was similar below.