Relationships between immunocompetence, somatic condition, parasitism and water temperature in a wild population of chub Leuciscus cephalus were investigated. The effects of a rapid temperature increase in early spring were studied for both sexes as water temperature affects immunocompetence. Investment in gonads and activity of mucus lysozyme were negatively correlated; lysozyme activity decreased as temperature increased. No correlations were found between lysozyme activity and parasitism or intensity of infection by monogeneans, the most abundant metazoan parasite group in L. cephalus. There was a positive correlation, however, between respiratory burst intensity and parasitism. Indices of investment in gonads and spleen were correlated, showing that energetic reserves allowed either investment in gonads and spleen, or that spleen investment, even if often used in other studies in immunoecology, was not always a significant indicator of immunocompetence during this period. This last proposition is supported by the lack of correlation between spleen investment and other factors linked to immunocompetence.