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The peritrich epibiotic ciliate Trichodina diaptomi Basson and Van As, 1991 was recorded on the body surface of two Neotropical species of the calanoid copepod genus Mastigodiaptomus in north-central Mexico. Specimens infested with the ciliate were collected from a pond near a large eutrophic reservoir. This ciliate was identified as T. diaptomi by its body size, the morphometry or shape of the taxonomically relevant structures such as the adhesive disc, macronucleus, and denticles or the modal number of radial pins, rays, and denticles. The Mexican specimens have a higher modal number of denticles with respect to other known populations of T. diaptomi. Ciliates were observed crawling freely over the copepod cephalothorax and attached at times, leaving a mark on the cuticle but causing no evident physical harm to the copepod. Our observations arise some doubt concerning the presumedly parasitic association of this species with copepods. This widespread species, currently known as a symbiont of 10 species of diaptomid copepods, was reported recently for the first time in the Americas, from Brazil. This is the second illustrated record of this epibiotic ciliate in the continent and the first in North America. Inglés