Aims: To investigate the fate of Aeromonas hydrophila pathogenicity when cells switch, in nutrient-poor filtered sterilized seawater, between the culturable and nonculturable state. Methods and Results: Aeromonas hydrophila ATCC 7966, rendered non culturable. within 50-55 days of exposure to marine stress conditions, was tested for its ability to maintain haemolysin and to adhere to McCoy cells. Results showed that pathogenicity was lost concomitantly with culturability, whereas cell viability remained undamaged, as determined by the Kogure cell elongation test. However, this loss is only temporary because, following temperature shift from 5 to 23degreesC, multiple biological activities of recovered Aer. hydrophda cells, which include their ability to lyse human erythrocytes and to attach and destroy McCoy cells were regained. During the temperature-induced resuscitation, constant total cell counts were observed. Moreover, no significant improvement in recovery yield was obtained on brain-heart infusion (BHI) agar plates amended with catalase. We suggest that in addition to the growth of the few undetected culturable cells, there is repair and growth of some mildly injured viable but nonculturable cells. Conclusions: The possibility that nonculturable cells of normally culturable Aer. hydrophila in natural marine environment may constitute a source of infectious diseases posing a public health problem was demonstrated. Significance and Impact of the Study: These experiments may mimic what happens when Aer. hydrophila cells are released in natural seawater with careful attention to the conditions in which surrounding waters gradually become warmer in late summer/early autumn.