Methylmercury (MeHg) is highly immunotoxic and can alter the health status of the harbour seal, Phoca vitulina, from the North Sea. To investigate the mechanism of MeHg-induced toxicity in harbour seal lymphocytes, Concanavalin A (ConA)-stimulated peripheral blood leucocytes were exposed in vitro to sublethal concentrations of MeHgCl (0.2, 1, and 2 µM) for 72 h and then analysed for their viability and ultrastructure. After 72 h of incubation, cells were counted with a propidium iodide staining technique, a metabolic MTS assay was performed, and cells exposed to 1 µM of MeHgCl were observed by transmission electron microscopy (TEM). Alive cell numbers decreased with increased MeHgCl concentrations. In presence of ConA and 1 µM of MeHgCl, TEM images revealed a higher frequency of apoptotic cells. Exposed cells displayed condensation of the chromatin at the nuclear membrane and mitochondrial damages. The results suggest that in vitro MeHgCl-induced apoptosis in harbour seal lymphocytes through a mitochondrial pathway.