"The presence of a member of the genus Heterobothrium is described for the fi rst time from the coast of Yucatán Peninsula, southeastern Mexico. Heterobothrium lamothei n. sp. is recorded parasitizing the gills of the tetraodontid fi sh Sphoeroides testudineus (Linnaeus, 1758) from 4 coastal lagoons in Yucatán: Celestún (20° 52’ N, 90° 24’ W), Chelem (21°15’N 89°45’W), Dzilam (21°35’N 88°35’W) and Río Lagartos (21°22’N 87°30’W). The new species differs from the other species described in the genus, by a combination of characters including a copulatory organ armed with 12-15 genital hooks, the distal pair of clamps smaller in comparison with the 3 other pairs and by having 15-26 testes. The infection parameters were for Celestun, 47 % of prevalence, number of fi sh examined (n) = 47, mean abundance of 1.76 ± 9.62; Chelem, 20 % (n = 30), 0.53 ± 8.48; Dzilam, 2 % (n = 60), 0.02 ± 0.00; Río Lagartos, 25 % (n = 59), 0.34 ± 1.58. Heterobothrium lamothei n. sp. may be considered as potentially dangerous species for the aquaculture of S. testudineus due to its direct life cycle hat high fi sh densities would increase the transmission of this monogenean."