The copy number of the rrn operon in 70 strains of Acinetobacter including the type strains of almost all the genomic species with validated names was estimated after digestion of their genomic DNA by the restriction enzymes BglII and PstI, and Southern blotting. Copy number estimates varied between and among species, with between 3 and 7 rrn operon copies detected. Copy number estimates obtained from the same strain with the two enzymes sometimes varied. BglII generated RFLP patterns of the rrn containing fragments obtained from Southern blots after agarose gel electrophoresis were examined for their value in identifying Acinetobacter isolates. This method was very reproducible with the same fragment pattern always generated from the same isolate on repeated analysis. Often multiple strains of the same genomic species gave identical or very similar patterns (e.g. Acinetobacter baylyi), clustering closest together on the dendrogram generated after numerical analysis of these patterns. However, with some, like BG5 and BG8, the patterns derived from the different strains, some of which had been placed in this genomic species from DNA:DNA hybridization data, varied considerably to each other and to the type strain. Little similarity was seen when relationships between these strains based on these patterns were compared to those using DNA:DNA hybridization data. Often these patterns could be used to question earlier identification of strains using phenotypic characters. Thus, strain AB82 thought to belong to genomic species 5 gave an identical pattern to A. bouvetii(T) (DSM 14964). In some cases this pattern analysis suggested that novel species of Acinetobacter might exist among the strains examined.