Laboratory studies were conducted to determine the effects of constant temperatures (7, 22, and 30°C) and corresponding fluctuating temperatures (0–14, 15–29, and 23–37°C) on the development of diamondback moth, Plutella xylostella (L.), and its North American parasitoid Diadegma insulare (Hellén). Parasitized third-instar diamondback moth larvae were reared until adult mortality in individual thermal gradient cells at different temperature regimes. Larval mortality, parasitism success, pupal mortality, larval and pupal developmental time, adult longevity, and pupal and adult dry weight were recorded. Overall diamondback moth larval mortality was low. The pupal mortality of D. insulare increased with increasing temperature; however, diamondback moth did not show such a response. Greatest parasitism success (67%) was found at constant and fluctuating 22°C and fluctuating 7°C, and the lowest (30%) at fluctuating 30°C. Longer development times and greater pupal body masses occurred at lower temperatures for both insects. Significant differences occurred between constant and fluctuating temperature regimes for most parameters of both insects. Fluctuating compared with constant temperatures caused shorter development times, similar body mass, and higher adult longevity for both insects at optimal and lower temperature ranges. Both insects experienced 0°C at fluctuating 7°C (0–14°C) and survived. These results have important implications for extrapolating temperature effects on insects in laboratory studies with constant temperatures. Comparing successful parasitism capacity of the wasp and pupal survival and body mass of both host and parasitoid, we conclude that D. insulare is a more effective parasitoid at lower temperatures.