Three species of hymenopterous parasites of the spotted alfalfa aphid were reared over a range of constant temperatures. At each thermal level, life—table data were obtained for each species. These data were used in computing certain statistics concerning reproduction and rates of potential population increase which are valuable in assaying the effectiveness of each species as an aphid parasite. The temperatures studied ranged from 10 to 35°C, and the relative humidity was held as constant as possible between 40 and 60%. Artificial illumination and photoperiods were identical in all studies. The studies showed that the braconid, Trioxys utilis Muesebeck, generally has the highest mean total fecundity of the three species, although the aphelinid, Aphelinus semiflavus Howard, produced more eggs in the temperature range of 18 to 22°C. The gross reproductive rate and net reproductive rate of T. utilis were also higher than the other parasites throughout most of the temperature range studied, although again A. semiflavus showed higher reproductive rates between 24 and 27°C. The innate capacity for increase, rm, of T. utilis was decidedly the highest of the three parasites at all temperatures. The braconid, Praon palitans Muesebeck, was inferior tot he other two parasites in nearly all phases of these life—table statistics. Its inclination to enter a facultative hibernal diapause at relatively mild temperatures, plus its intolerance of higher temperatures, which the other two species endured, limited its thermal range of effectiveness. Statistics such as total fecundity, gross reproduction, rate, net reproduction rate, and innate capacity for increase are discussed relative to their value in indicating the potential effectiveness parasites can be expected to possess in the field.