Correlations between body size and latitude or altitude are a common phenomenon both within and among animal taxa. We present data demonstrating an unusual longitudinal cline in adult size of lubber grasshoppers (Romalea microptera) in sub-tropical Florida. We tested whether this longitudinal size cline could result from variation in any of three life history traits: size at hatching; growth rate during development; and duration of nymphal development. Field surveys indicated that adult body size (thorax length, femur length) of both males and females increases along a west-east cline. A laboratory study indicated that size at hatching did not differ between one large-bodied (eastern) population and one small-bodied (western) population. Field surveys of nymphal growth and development in six populations indicated that growth rate (mass increase per day) was higher in two small-bodied (western) populations relative to all other populations. Nymphal development was faster in small-bodied (western) populations relative to large-bodied (eastern) populations. However, our survey could not separate the potential effect of inter-population differences in development time from inter-population differences in hatching time. Our data suggest the longitudinal size cline could be caused by inter-population differences in development time and that growth rate and size at hatching play less of a role in producing the longitudinal cline. Future work will test the roles of natural selection and neutral processes (e.g., genetic drift, dispersal) in producing the longitudinal size cline.