Phenanthrene, a major component of crude oil, is one of the most predominant polycyclic aromatic hydrocarbons (PAHs) in aquatic ecosystems. The present study aimed to assess the in vitro effects of phenanthrene on the clam, Ruditapes decussatus. For this purpose, haemolymph samples were exposed for 24 h to two concentrations (10^-3 and 10^-5 mg mL^-1) of this pollutant. Immune function was assessed using a combination of humoral and cellular responses. The toxic exposure caused alteration in the immune parameters of clams with a significant increase (ANOVA, p<0.05) in total haemocyte count and haemocyte mortality and a significant decrease (ANOVA, p<0.05) in lysozyme and esterase activities. This study investigates the direct effects of phenanthrene on humoral and cellular immune functions in clam Ruditapes decussatus. Further research is necessary to clarify the immunomodulatory effect of this contaminant on the whole animal.