Based on morphological observations carried out on the same material, but submitted to different processing,Trithyrodinium fragileDavey 1969 is herein demonstrated to be a junior synonym ofTrithyrodinium evittiiDrugg 1967. From both original descriptions, the only way that the two species can be separated is by the presence or the absence of a thick brown organic layer on the surface of the endocyst. The present study proves that this is an artificial separation caused by different processing techniques. As a result,T. evittiiis emended in order to encompass specimens, wholly or partially exhibiting this brown organic layer and previously attributed toT. fragile.