The terrestrial hydrophilid genus Agna Smetana (tribe Megasternini), specialized in rotten cacti, is redescribed and illustrated. Known species are diagnosed and a new one, A. zaragozai sp. nov., is described from central Mexico (Hidalgo, Puebla, and Oaxaca) and its molecular barcode is provided.
Other species of Hydrophilidae known to have been collected in cacti are listed and commented. Dactylosternum cacti (LeConte) (Coelostomatini) is recorded for the first time from Mexico and Cryptopleurum impressum Sharp (Megasternini) is recorded for the first time from the Mexican states of Jalisco and San Luis Potosí.