Though many studies have been carried out on mortality events in Crassostrea gigas this is the first study of the phenomenon in the Irish Sea. Unexplained summer mortalities have been taking place in C. gigas populations in Ireland since the 1990's. Production of this species currently occurs at low levels in Wales, with no reports of unusual mortalities and, as similar environmental factors and husbandry techniques exist in both countries, a comprehensive investigation into mortalities in Ireland was considered to be beneficial to the industry in that country but applicable to growers in Wales also. A single cohort of C. gigas was obtained from a hatchery in both 2003 and 2004, and performance was compared over the summer months of both years following relaying at sites in Ireland and Wales. Oyster growth, condition, biochemical content and gonad development were examined and the influence of these parameters on summer mortality was investigated. Oysters were sampled every second week. Significant mortalities occurred at the Irish sites during the study but not at the Welsh sites. Oysters at the Irish sites displayed significantly increased growth, condition and gonad development compared to oysters at the Welsh sites. Both protein and glycogen levels were higher in the oysters held at the Irish sites than in the Welsh sites. Differences in the measured parameters between the two sites were greater in 2003, when temperatures were higher, than in 2004. Fast growth rates, gonadal development and spawning may contribute to increased physiological stress in young oysters making them more susceptible to mortalities during the summer months. This study indicated that site-related differences, such as environmental parameters, have a significant effect on subsequent performance of oysters and, may strongly influence mortality rates. (C) 2010 Elsevier B.V. All rights reserved.