Life-cycle theory suggests that smaller reproductive propagules should be produced in environments in which juvenile growth rate is higher. It has recently been shown that this holds both if adult size is fixed and if the timing of breeding is fixed. Here we test the prediction qualitatively in natural populations of Theoxodus fluviatilis (L.). Capsule size and juvenile growth rate were measured at five sites in southern England. Significant differences between sites were found in both characters. Larger capsules occurred at the sites with lower growth rate, as predicted by life-cycle theory.