Vibrio aestuarianus is frequently found in coastal areas and can infect and induce mortalities in the pacific oyster Crassostrea gigas. However, nothing is known about its distribution and seasonality in the estuarine environment, especially where oyster farming is practiced. Its occurrence was investigated in sediment and oyster haemolymph at 2 oyster farms in Brittany (France) over 2 yr during 2 periods, from June to September 2007 and from February to June 2008. Total heterotrophic bacteria (HB) were cultured on marine agar while total Vibrio spp. and V. aestuarianus were selectively numerated using thiosulfate citrate bile salts sucrose agar (TCBS agar) and the species-specific hybridisation method, respectively. PCR was performed to detect V. aestuarianus in sediment when it became unculturable. Both total Vibrio spp. and V. aestuarianus had a seasonal trend. The highest concentrations were recovered in the warmest months. Its abundance ranged from 10(2) to 4 x 10(5) CFU ml(-1) in haemolymph and from 10(3) to 1 x 10(4) CFU g(-1) in the sediment. Temperature was the main factor influencing the concentration of Vibrio spp. and V. aestuarianus in the sediment. Thus V aestuarianus might subsist during the cold seasons in the sediment, from which it can emerge when environmental conditions became favourable.