Standardized laboratory bioassays were carried out at the Institute for Biological Control of the Federal Biological Research Center for Agriculture and Forestry to determine the susceptibility of two Colorado potato beetle (CPB) populations to Bacillus thuringiensis ssp. tenebrionis (B.t.t.). The beetles were collected from fields near Darmstadt/Hessen, and Michelfeld/Baden-Württemberg. The Michelfeld population, collected from an ecological farm, was sprayed since 1993, once or twice a year with NOVODOR FC, a formulation of B.t.t. The Darmstadt population was never sprayed with B.t.t. before. Our studies showed that the susceptibility of the larvae (LD 50 values) of the two populations did not differ each other significantly. Although resistance of CPB to B.t.t. under laboratory conditions has already been demonstrated, our presumption is that the resistance development of CPB on the field is probably much slower when the farmer is using crop rotation.