Parahaploposthia thiophilusis described as a new species of the acoel turbellarian family Haploposthiidae. AlthoughP. thiophilusexhibits strong anatomical similarities to other members of the genusParahaploposthia, notably in the male copulatory organ, the male gonads and the frontal organ, this new species differs from all other members of its genus by the possession of a bursa. We have broadened the generic diagnosis ofParahaploposthiato include this feature, a measure that we prefer to the erection of a new genus based on a character (the bursa) of uncertain validity in the present system of acoel taxonomy. This species description is based primarily on information gleaned from the observation of living specimens by phase contrast microscopy, in order to illustrate the utility of an often neglected method that reveals important information not available from fixed and sectioned specimens, and which also may provide for more rapid and simple identification of acoel turbellarians by nonspecialists.