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The salp Iasis zonaria is often found over the Argentine continental shelf of the southwestern Atlantic Ocean and, occasionally, its high densities dominate over other groups of zooplankton. Therefore, a better understanding of the basic aspects of its life history is essential for understanding the mechanisms underlying bloom formation. In this study, I. zonaria was collected during the austral winters of 1999, 2000 and 2001. Additional data were obtained from this area from May to November 1978. The most widespread distribution and largest catches occurred in 1999 (≈55 100 ind. 10 m−2). Densities decreased markedly in 2000 and the species was virtually absent in 2001. The growth of the solitary stage blastogenetic stolon and formation of several blocks of aggregates buds are described. Eight development stages were characterized and it is estimated that each solitary can produce at least four blocks and a total of ≈ 420 aggregates. The average number of buds per block increases from the older (first formed) to the youngest. The total number of buds (y) was related to solitary length (x) as: y = 4.94 x – 134.57. Four developmental stages were described for aggregate individuals. Based on monthly samples collected in 1978, the estimated lifespan of aggregate individuals varies between 11 and 14 months.