High-performance thin-layer chromatography (HPTLC) analysis was done on lutein and β-carotene in the digestive gland–gonad complex (DGG) and whole body of uninfected Cerithidia californica snails and those infected with the larval trematodes Mesostephanus appendiculatis or Euhaplorichis californiensis. HPTLC of the DGG extract on C-18 reversed-phase plates developed in petroleum ether–acetonitrile–methanol (1:2:2) mobile phase showed 2 identifiable pigment zones; the least polar zone had a retention factor (R_f) of 0.07, identical to a β-carotene standard, and the more polar zone had an R_f of 0.41, identical to a lutein standard. Densitometric scanning of the pigment zones in sample versus standard chromatograms showed that the weight percent of lutein in the uninfected DGGs (3.4 × 10⁻³%) was significantly greater (P < 0.05) than that of DGGs infected with either M. appendiculatis (0.35 × 10⁻³%) or E. californiensis (0.82 × 10⁻³%). Changes in β-carotene in the infected DGGs were insignificant compared to the uninfected controls. However, the β-carotene content of whole snails was significantly reduced (P < 0.05) by infection with either trematode.