Vegetative reproduction by means of abscised fragments was studied in the introduced red alga ‘‘Heterosiphonia japonica’’. Small side branches (pseudolaterals) are actively shed in all seasons, but mostly during late summer/autumn. The development of excised pseudolaterals was studied in the laboratory under different culture conditions. At 08C, few pseudolaterals survived and no development was observed. At 48C, survival was high but development was slow, while at 12 and 208C the majority of pseudolaterals developed rhizoids and attached to the substratum. Aquarium studies of fragmentation rate of pseudolaterals and estimates of tetraspore production showed that the instantaneous fecundity by fragments could be almost as high as the estimated fecundity by tetraspores. Field observations showed recruitment of ‘‘H. japonica’’ by fragments during all seasons. There was a weak trend of higher densities of recruits originating from fragments during late summer, consistent with the high degree of pseudolateral shedding and optimal temperature for pseudolateral development at this time of the year. The results from this study show that the newly introduced species has a high capacity for regenerating from small branchlets, and a continuous release of these vegetative propagules throughout the year.