Sex-change (protandry or protogyny) has gone unnoticed in many bivalves due to the rapid occurrence of this process. In this paper, histological evidence and analyses of the size structure of three geographically separated populations on the northwestern coast of México together with theoretical considerations strongly suggest that Atrina maura (Sowerby I, 1835) is a protandric species. The gonads of a total of 931 A. maura specimens collected in the three study sites were analyzed: Ojo de Liebre lagoon (27°55′N, 114°20′W from March 2002 through February 2003), Ensenada de La Paz (24°11′N, 110°26′W, from May 2004 through June 2005 and during 2007), and Bahía Magdalena (24°30′N, 111°48′W, from March through October 2008). Microscopic analysis of gonads of hermaphrodite specimens enabled the reconstruction of a detailed histological sequence during the male-to-female transition. Analysis of the sex ratio by size class revealed that males occur primarily in the smaller classes, females in the larger classes and hermaphrodites in intermediate size classes. All the evidence substantiate the hypothesis that this species is not gonochoric.