The pollen morphology of the six SwedishCallitricheL. species was investigated using LM, SEM and TEM. The surface ornamentation is reticulate inC. stagnalis, C. platycarpa, C. cophocarpaandC. palustriswith supratectal elements in the first three species. The ectexine is reduced inC. hamulata, occurring as scattered granules, but completely absent inC. hermaphroditica. None of the species has distinct apertures butC. cophocarpaandC. platycarpaboth have leptomata. The ultrastructure of the pollen wall inC. palustrisshows regions with lamellated footlayer and an endexine which may function as apertures. Pollen morphology is found to be related to pollination biology; species with leptomata sometimes being pollinated through ephydrophily and species with reduced exine being hyphydrophilous.