The supernatant arising after biomass separation of Bacillus thuringiensis var. israelensis by flocculation/ sedimentationwas re-used after being supplemented with 25, 50 and 75% (w/v) of the original culture medium, based on corn steep liquor, glucose and mineral salts. Supplementation at 75% gave a spore concentration (1 x 1010 c.f.u. ml(-1)) five times greater than that obtained with the other supplements.