Abstract Antennal sensilla are documented for the first time in the family Helotrephidae. Sensilla in Hydrotrephes sp. include three forms of trichoid sensilla and two forms of leaf-like sensilla. Most of the sensilla are located on the apical two thirds of the distal segment. Morphology of the antenna in Helotrephidae appears to be of taxonomic use to define genera and it may be useful to determine relationships between genera within Helotrephidae and between the families Helotrephidae and Pleidae.