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Peritrichs usually settle on the external surface and gills of crustaceans. However, their physiological effect on the host has not been studied. Respiratory rate is a susceptible index for some factors that affect an organism. To test if ciliates attached to the crustacean gills have a physiological effect, we measured crustacean oxygen consumption in a closed system. Crayfish <i>Cambarellus</i> (<i>Cambarellus</i>) <i>montezumae</i> were collected in Lago Xochimilco in Mexico City and were isolated in chambers. Two peritrich species, <i>Cothurnia variabilis</i> (32.5%) and <i>Epistylis gammari</i> (67.5%), were observed on the gills. The metabolic rate was evaluated by oxygen consumption with a polarimetric oximeter. Statistical analyses demonstrated that a density of ciliates attached to the gills of the crayfish above ~50 individuals increased oxygen consumption with a positive correlation to epibiont density. We conclude that <i>C. variabilis</i> and <i>E. gammari</i> ciliate epibionts attached to the gill of the crayfish <i>Cambarellus</i> (<i>Cambarellus</i>) <i>montezumae</i> cause an increase in metabolic rate.