Acinetobacter strains use hydrophobic carbon sources and most of them are efficient oil degraders. They secrete a variety of emulsifiers which are efficient in producing and stabilizing oil-in-water emulsions. The bioemulsifier of Acinetobacter radioresistens KA53 (Alasan) is a high-mass complex of proteins and polysaccharides. The major emulsification activity of this complex is associated with a 45 kDa protein (AlnA), which is homologous to the outer membrane protein OmpA. The emulsification ability of AlnA depends on the presence of hydrophobic residues in the four loops spanning the transmembrane domains. The finding of a secreted OmpA was unexpected, in view of the fact that this protein is essential in all Gram-negative bacteria, has four trans-membrane domains and is considered to be an integral structural component of the outer membrane. However, secretion of an OmpA with emulsifying ability could be of physiological importance in the utilization of hydrophobic substrates as carbon sources. Here we examined the possibility that secretion of OmpA with emulsifying activity is a general property of the oil-degrading Acinetobacter strains. The results indicate that OmpA is secreted in five strains of Acinetobacter, including strain Acinetobacter sp. ADP1 whose genome has been sequenced. The ompA genes of ADP1 and an additional strain, Acinetobacter sp. V-26 were cloned and sequenced. Structure analysis of the sequence of the two proteins indicated the existence of the hydrophobic regions, previously shown to be responsible for the emulsification activity of AlnA. Further examination of the recombinant OmpA proteins indicated that they are, indeed, strong emulsifiers, even when produced in Escherichia coli. The finding that Acinetobacter OmpA has emulsifying activity and that it is secreted in five strains of Acinetobacter may be physiologically significant and suggests the involvement of this protein in biodegradation of hydrophobic substrates, including hydrocarbons.