A microcomputer based system for the identification of unknown isolates of Bacillus species is described. The identification matrix includes 78 test probabilities for 38 recognised species and other groups in the genus Bacillus and it is based on the work of Logan and Berkeley (1984). Morphological characters together with the results of tests using API 20E and API 50CHB, read after 24 and 48 h incubation, are used to obtain a probabilistic identification of an unknown aerobic endospore forming rod. Any differences between the observed and expected results for any identified organism are listed. Identification can be attempted on the basis of a limited set of test results, although this is rarely if ever done with this largely API based system, and if the unknown cannot be successfully identified then a set of additional tests can be selected which should permit identification. The computer system can store and recall test results entered for any isolate. This feature allows the accumulation of data on isolates which could be used to update the identification matrix in future taxonomic studies.