We examined the relative growth of threespine sticklebacks collected from 11 lowland streams in southeastern England during late autumn to test for between-catchment variations as well as development-related shifts in morphology. And, to understand better the sympatric relationship between threespine Gasterosteus aculeatus and ninespine Pungitius pungitius sticklebacks in small lowland streams, we compared growth variability in 35 mensural characters and morphological indices related to functional morphology in adults from one of these sites. Threespine demonstrated great plasticity, with some morphological differences observed between stream catchments, mainly in ventral spine and dorsal spine length, pre-anus and preanal distances, fin depth, and ventral body width (represented in the basipterygium medial plate width). Developmentally, three groups of mensural characters were found: 18 characters best explained by a linear regression (isometric growth), 9 characters best explained by a quadratic equation (gradual allometry), and 8 characters best explained by a split-linear regression (mainly isometric growth, with an abrupt shift in proportional growth – between 40 and 58 mm SL). However, these shifts did not appear associated with common ecological, physiological and/or behavioural functions. Compared with ninespine stickleback, threespine has a more robust body with stronger and more prominent spines, shorter tail, and eyes situated more backwards. Functional morphological indices suggest threespine to be a slower but more manoeuvrable swimmer than ninespine, which acquires a relatively more cruiser-form shape with reduced drag and more lift.