A laboratory experiment was carried out to isolate and study the effect of different concentrations of Bacillus thuringiensis (Bt) crude protein on Diamond back moth (Plutella xylostella). Crude protein from thirteen Bt isolates was extracted and used at different concentrations viz., 100, 250, 500, 750, 1000 and 2000 ppm for bioassay against third instar larvae of P. xylostella. The per cent mortality was recorded after 24, 48 and 72 hrs of treatment. A cumulative mortality of 100 per cent was recorded by three isolates viz., UK-13C, UK-762D, UK-25A as well as by the reference strain HD1, followed by UK-52A, UDP-420B, DK-45B and DK-6B (93.00, 86.67, 86.67 and 83.33 respectively) at 2000 ppm after 72 hrs. The crude protein of other isolates caused larval mortality in the range of 66.67 to 80.00 per cent at 2000 ppm after 72 hrs. No larval death was observed in the control. Irrespective of the crude protein concentration, the larval mortality was highest on third day, followed by second day.