The Lagos lagoon is continually being subjected to all forms of pollution. Human activities like industrialization, agriculture, and oil exploration are constantly threatening the health of this aquatic ecosystem. Polycyclic aromatic hydrocarbons (PAH) and mono-aromatic hydrocarbons-benzene, toluene, ethylbenzene and xylene (BTEX) are chemicals derived from fossil fuels that easily get into the aquatic environment. These chemicals were investigated in the fish parasite Tenuisentis niloticus, as a reflection of the extent to which these compounds are present in the lagoon. One hundred (100) samples of Heterotis niloticus were investigated for parasitic infection for a duration of six months, while the parasites encountered were used as sentinel organisms to check for the presence of PAH and BTEX. GC-MS was used to analyse for the presence of these chemicals, while parasite identification was done using conventional method. Only one parasite species, Teneuisentis niloticus, an acanthocephalan was recovered. There was 72% infection prevalence. The result also revealed that both BTEX and PAHs were accumulated in the body of the parasite. Two components of BTEX, (1,2-dichlorobenzene and 1,3-dichlorobenzene) and five PAH congeners-Acenaphthylene, Phenanthrene, benz(a)anthracene, benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene were not detected in the parasite. Total BTEX and PAH concentrations in the parasite was 47.45±7.68 and 135.12±2.547, respectively. There are ecotoxic concerns in the surroundings of the lagoon and human activities that aid it should be checked.