Of the six subfamilies in the family Naucoridae, Cheirochelinae is the only subfamily for which immature stages have not been described. This work represents the first contribution of nymphal instar descriptions in the subfamily Cheirochelinae. Herein, the five instars of Gestroiella siamensis Polhemus, Polhemus, and Sites are illustrated and described. The descriptions are based on specimens collected in Ton Nga Chang Wildlife Sanctuary in southern Thailand. The primary characters to distinguish among the instars are overall body size and relative lengths of the mesonotal wing pad compared to the exposed lateral margin of the metanotum. A phoretic association between nymphs of G. siamensis and larvae and pupae of the chironomid Nanocladius (Plecoteracoluthus) sp. also is documented and discussed.