A study is presented of the patterns of growth in Devonian fossil Rhipidistia, particularly Eusthenopteron foordi Whiteaves. Significant changes in basic anatomical characteristics including certain cranial proportions and the relative position of insertion and shape of the fins may be correlated with differences in standard length. These differences result from various types of differential growth. Growth markings on the scales indicate that the“juvenile”condition may last through two seasons of growth. The differences in body configuration are interpreted as being fully adaptive and related to important differences in locomotor patterns and ecology. The results of the study suggest that the rhipidistian fish formerly known as“Thursius ? clappi”Romer is a juvenile and a re-interpretation of this form is given.