Cromartie was considered something of a defensive liability his rookie season, and worked on his defense during the off season. As a result, he led National League left fielders in many defensive categories in . He, Dawson and Valentine each led their respective positions in outfield assists to give the Expos the unquestionable top defensive outfield in the major leagues. With his bat, he got off to a slow start in 1978. He ended April with just a .193 batting average and two RBIs. From there, he batted .307 over the rest of the season to finish just below .300 (.297, which was tops on the team). His first major league grand slam was a walk off against the Atlanta Braves on July 19.