Effects of organophosphorous pesticide, dimethoate on blue mussels, Mytilus edulis using selected biomarkers have been studied. Mussels were exposed to serial dilutions of dimethoate, 7.88, 15.75, 31.35, and 63.00 µg/l including positive and negative controls for 14 days. The suppression effects of dimethoate on phagocytic activity significantly occurred at two lowest concentrations of dimethoate (7.88 and 15.75 µg/l), but stimulation effects significantly emerged at the following highest concentrations (31.35 and 63.00 µg/l). The declining tendency of the cholinesterase (ChE) activity (23% lower than the control) appeared when mussels exposed to 7.88 and 15.75 µg/l dimethoate. Moreover, the significant inhibition of the ChE activity occurred at 31.35 µg/l dimethoate exposure. This study suggested that the phagocytic and the ChE activity are useful biomarkers for assessing the affects of organophosporous pesticide, dimethoate on neuro-immune system of blue mussels, M. edulis.