High summer temperatures are one of the most stressful environmental problems confronted by the abalone mariculture industry and are commonly associated with outbreaks of infectious disease. We tested the effect of extreme but non-lethal elevated temperatures on abalone immunology, biochemistry and quantitative histology. We subsequently compared the haemolymph results to the histology to gain increased understanding of how heat stress impacts abalone health. Abalone were kept in water that was heated from the ambient 16. °C temperature to 26. °C within 5. h and then held at 26. °C for one week to determine the effects of this acute heat stress on the day of temperature elevation and whether there was acclimatization or deterioration 2 and 7. days later. Antibacterial activity, phenoloxidase activity and neutral red retention times declined significantly with heat and did not recover. The total haemocyte count was elevated significantly during heat stress and was highest on day 1. The phagocytic rate was elevated on day 1 but had recovered by the following day. Acid phosphatase activity, leucine aminopeptidase, haemolymph protein and haemolymph electrolytes (calcium, phosphate, magnesium, sodium, and chloride) were not significantly affected by heat stress. This indicates that severe heat stress causes changes in some, but not all haemolymph parameters. The sublethal immunologic effects seen in haemolymph samples occurred concurrently with histological changes. The digestive gland had significantly increased haemocyte infiltrates in heat stressed abalone. Heat stressed abalone had significantly greater loss of epithelium lining from the gills, with no recovery. The gill goblet cell numbers declined significantly on day 2 and had recovered by day 7. There was no significant change in the volume of fluid or protein concentration of the haemolymph in the gill sinuses between treatment groups. These results indicate that immunosuppression and organ damage are likely to be involved in the increased incidence of bacterial disease reported by abalone farmers during summer