A study on strains received as Pseudomonas alcaligenes was performed by the DNA-DNA hybridization procedure. Results on strains phenotypically distinct (subcluster A1; Gavini et al., Int J Syst Bacteriol [1989] 39:135-144) from P. alcaligenes strains (subcluster B1) pointed out that (a) these strains belonged to Comamonas testosteroni species (DNA-DNA homology 75%-98%) and (b) the two species could be differentiated by biochemical tests other than type of flagellation or poly-beta-hydroxybutyrate deposits.