Abstract: Lysosomal biomarkers are early warning signals of the biological effects caused by environmental pollutants. However, no studies have examined the effects of petroleum contaminant on lysosomal integrity in the clam Ruditapes philippinarum. Dose and time responses of three lysosomal biomarkers from the clam to 0# diesel oil water soluble fraction (WSF) were studied in this paper. After the clams were exposed to the diesel oil WSF at doses of 0.1～1.6 mg/L for 5 days, neutral red retention time (NRRT) in haemocytes of the exposed groups showed significantly lower than those of the control groups by 23.1% to 48.5% (p<0.05 orp<0.01), and the acid phosphatase (ACP) activities in gill and viscera of the exposed groups showed no significant difference compared to those of the controlled. Furthermore, when the clams were exposed to the WSF at a concentration of 0.8 mg/L for 11 days and the lysosomal biomarkers were analysed on days 0,1,3,7 and 11. The NRRTs in haemocytes of exposed groups were consistently lower than those of the control groups except that a significant difference occurred between two groups on days 1 and 3 (p＜0.05). ACP activities in the gills and viscera of exposed groups increased gradually to a maximal level on day 7 and then declined rapidly. From day 3 to day 11, ACP activities in the gills of exposed groups were significantly different from those of the control groups (p<0.01). From day 7 to day 11, ACP activities in the viscera of exposed groups were significantly different from those of the control groups (p<0.05). No response was found in lysosomal membrane stability (LMS) test for the reason that no positive histochemical staining reaction of N-acetyl-β-hexosaminidase appeared on frozen sections of the digestive gland of the clam. The present results indicate that NRRT in haemocytes and ACP activity in gills of the calm can be applied as potential biomarkers for monitoring biological effects of pollutants in marine environment.