The molecular mass of different Cry toxins produced by Bacillus thuringiensis bacteria was estimated by size-exclusion chromatography and non-denaturing polyacrylamide gel electrophoresis at neutral and alkaline pH in order to assess the existence of oligomers in solution. We found that Cry1Aa, Cry1Ac, Cry1C, Cry1D and Cry3A toxins exist in solution as a mixture of monomer and high molecular mass aggregates with an apparent molecular mass greater than 600 kDa, that depend on the time elapsed between toxin activation and analysis. Aggregation of toxins by disulfide bonds is unlikely because aggregates are also observed in samples incubated with DTT. These data show that the Cry toxins studied do not form oligomers of less than ten subunits in solution and suggest that oligomer formation may occur after the toxin binds to the receptor and inserts into the membrane.