The appearance of Mesocyclops leuckarti (Claus) in Lake Kinneret was found to be as a population with mixed age structure present throughout the year, when average epilimnic temperatures varied between 14 to 28°C. Some Physiological features were studied, at three temperatures (15, 22, and 27°C), in order to give an insight into such an occurrence: the development time of instars, egg production, longevity, ammonia excretion, food intake and oxygen consumption of adult females. It is suggested that the high rate of metabolism and respiration and smaller body size at 22-27°C is compensated for by the higher rate of development of the offspring and egg production of the adult females.