Sporulation of Bacillus sphaericus strain 2297 in a synchronous liquid culture was studied by electron microscopy. The t0 of sporulation occurred 7 h after the beginning of the lag phase. Crystal-like inclusions first appeared at t2 and reached their final size between t5 and t6. The release of the spore/inclusion complex occurred at about t15 (22 h after inoculation). Toxicity against Culex pipiens larvae was related to sporulation and appeared during the early stages of sporulation. The LC50 (24 h) decreased about 10(5)-fold between t0-2 and t7, in correlation with the formation of crystalline inclusions. Heat resistance of spores appeared later than toxicity.

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A screening of the larvicidal activity of the more than 900 strains of Bacillus thuringiensis strains, combining the Institut Pasteur collection was realized. A quick bioassay using 1st instar larvae and semi-synthetic medium was developed.Many strains were toxic to Spodoptera littoralis, but only a few belonging mainly to serovaxs aizawai, kenyae and entomocidus showed high level of toxicity.The profiles of strain activities differed from serovar to serovar, but within the same serovar toxicity can vary with different strains. One aizawai strain tested in the field gave satisfactory results, better than a commercially used strain, tested in the same experiment. (1981) reported on the successful control of S. littoralis by strains of B. thuringiensis using powders of several strains, but the conclusion as to which was the most potent strains was not clear. In consequence we thought that by screening a large number of strains, we would have a fairly good chance to find strains sufficiently virulent to S. littoralis for field treatments. KEY-WORDS :

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