Laboratory studies were conducted on certain aspects of biology of Diadegma semiclausum Hellen, a larval parasite of a crucifer pest, Plutella xylostella (L.). Within the range of 15 degree C to 35 degree C, the higher temperature, the shorter was the duration of larval and adult stages. Egg hatching and adult emergence were high at 15 degree C to 30 degree C but were significantly reduced at 35 degree C. The higher the temperature, the higher was the proportion of males produced. Temperature threshold was 5.74 degree C for eggs, 3.80 degree C for larvae, 5.91 degree C for pupae and 6.60 degree C for adults. D. semiclausum oviposition in the first three P. xylostella larval instars produced more parasite males than females but oviposition in the fourth instar produced significantly more females than males. Parasite adults tended to emerge from their pupae from 06:00 to 09:00 hours although some emerged at other hours during the photophase. Adult longevity and production of eggs increased when adults were provided with a food source (honey) compared with no food or provision of water alone. Parasite adults survived and laid eggs for 28 days when provided with food but for only three days when deprived of food.