Summary In order to examine the efficiency of crossed immunoelectrophoresis (CIE) for the differentiation of species and for the determination of taxonomic relationships within the genus Bacillus, an investigation including strains of 20 species was performed. Ultrasonic extracts (USE) of sporefree-grown vegetative cells were used. With rabbit antisera against USE of type strains, the CIE was accomplished in homologous and heterologous combinations. USE's of additional strains were included into this investigation. In order to evaluate the immunoelectropherograms, the number of precipitates was counted. A mean of 103 precipitates was found in homologous reactions. With few exceptions, heterologous combinations showed less similarity in the number of antigens. Because of their high cross-reactions, the following species could not be differentiated: B. subtilis from B. amyloliquefaciens and strains of B. coagulans 1 1 Having completed the manuscript we felt the necessity to re-examinate the identity of the B. coagulans strains used in this examination. The results deny the exsitance of a close relationship between the two species B. coagulans and B. subtilis. They are summarized in the article “Represent the Batches 2 of Strain NCTC 3991 and NCTC 3992 B. coagulans?” Zbl. Bakt. Hyg. A 263 (1987) 525–529. , furthermore, B. cereus from B. thuringiensis. The different species revealed lower numbers of precipitates with decreasing taxonomic relationships in heterologous combinations. This observation was used to classify the investigated species by a ‘position-frequency analysis’ (PFA). After sorting the matrix of precipitate numbers in the ascertained optimal sequence of species, a cluster analysis was carried out. The phenogram showed 6 (respectively 8) group clusters. The members of the morphologic group I ( Smith et al., 1952) was found only in group cluster 2. The phenogram was partly in agreement with phenograms based on other characteristics, e.g. DNA hybridization.