Laboratory investigations were conducted on the biology of Apanteles subandinus Blanchard and the effect of temperature and relative humidity on its bionomics. Temperature responses of this parasite were compared with those of Apanteles scutellaris Muese-beck and their common host, the potato tuberworm, Phthorimaea operculella (Zeller). Responses of the parasites and their host to physical factors were evaluated by using the intrinsic rate of natural increase (r) as a bioclimatic index. At 80°F and 50 per cent relative humidity (R.H.) the mean duration of the life cycle of A. subandinus from egg to adult was 15 days; egg, 1 day; first in-star, 4.5 days; second instar, 2.5 days; third instar, 2 days; prepupa, 0.5 days; and pupa, 4.5 days. The parasite egg is deposited at random in the body cavity of the host larva where the parasite larva develops. There are three instars, the first being mandibulate-caudate and the other two hymenopteriform. The mature larva emerges from the host, killing it in the process, and spins a silver-white cocoon. There is essentially no preoviposition period in A. subandinus and the parasite is an arrhenotokous species, the virgin female producing only male progency. Between 60 and 90°F, speed of development was directly related to temperature. A constant temperature of 95°F prevented development of the parasite beyond the first instar. Longevity and reproductive periods were inversely related to temperature, longevity following the pattern of the curve of physiological longevity. Production of progeny was maximum at 80°F. As determined by calculations of the intrinsic rate of natural increase, A. subandinus persisted and increased in numbers between 60 and 90°F. Optimal temperature for the parasite was 85°F. Relative humidity did not affect development. At 80 and 85°F, maximum progeny production occurred at 50 per cent R.H. This level of humidity was also optimal for population growth. Apanteles scutellaris developed and reproduced at 95°F, a temperature at which A. subandinus could not develop. The optimal temperature for A. scutellaris was 90°F. The potato tuberworm host developed at 95°F but this temperature caused male sterility, preventing production of progeny. The shortest developmental period of the potato tuberworm occurred at 90°F. Comparisons of the temperature responses of the two parasites and their host revealed that the parasites have a higher power of increase than the host, and that A. scutellaris is better adapted to high temperatures than is A. subandinus.