Global warming is expected to increase nocturnal environmental temperatures. We determined how elevated nocturnal versus diurnal temperatures alter fitness components in the imported cabbageworm (Pieris rapae L.). Full-sib families of P. rapae caterpillars were split into control, increased nocturnal, and increased diurnal temperature treatments. We found significant variation among families for relative growth rate, development rate, and pupal mass. Some families had a positive correlation between pupal mass and development rate, whereas others had a negative correlation. On average, we found a faster development rate caused by the nocturnal treatment and a smaller pupal mass caused by the nocturnal and diurnal temperature treatments. We found no significant effect of the temperature treatments on relative growth rate. An exponential growth model suggests that, despite the decreased pupal mass associated with the increased nocturnal temperature regimen, the faster development rate would cause the nocturnal population to multiply more quickly than the control or diurnal populations.