The polymorphism of glutamate, lactate and malate dehydrogenases, of acid phosphatase and of esterases of 27 strains of Providencia alcalifaciens, 35 strains of P. stuartii and 17 strains of P. rustigianii was investigated by conventional electrophoresis in polyacrylamide agarose gel and by isoelectric focusing in thin-layer polyacrylamide gel. For each enzyme analysed, the three species were characterized by a distinct electrophoretic pattern. The number of allozymes detected by conventional electrophoresis was greater than that detected by isoelectric focusing. The use of these two techniques in parallel led to improved detection of polymorphism of esterase alpha beta from P. alcalifaciens. A two-dimensional profile obtained by plotting isoelectric points against electrophoretic mobilities for malate dehydrogenase, acid phosphatase and beta-A esterase showed a molecular relationship between the diverse allozymes and demonstrated their taxonomic values. Polymorphism varied considerably according to the enzyme and species analysed and was correlated with DNA heterogeneity. The strains of P. alcalifaciens exhibited the greatest enzyme polymorphism and were classified into two main zymotypes reflecting genetic divergence within this species, whereas the strains of P. stuartii were electrophoretically less variable.