Kingiella chilenica, a brooder mollusc, inhabiting soft bottoms of estuarine tidal flats of southern Chile, is well suited to study larval recruitment. It is a tiny semelparous clam with an annual cycle, and whose recruits must survive an inhospitable winter season prior to growing and reaching reproductive maturity in the following summer season. The population dynamics of this clam was studied through periodic sampling over two successive years to follow fluctuations in its' abundance. Obtained data on embryo production and recruit survival over both periods shows that population abundance varied widely between the two years of study. A high level on recruit survival through the winter of the first year resulted in high numbers of adults in the corresponding summer reproductive season. An inverse situation occurred in the second year, with lower recuit survival and, consequently, fewer reproducing adults. However, the lower abundance of adults in the second year was compensated by their having a higher survival rate over the reproductive season as compared with adults of the previous year. Independently of the abundance of the adults, the number of embryos incubated per female was very similar between both periods. Consequently, we suggest that the net contribution of juveniles produced by females per unit substrate was similar between the annual cycles studied. As predicted for a semelparous species (high reproductive effort), the maximization of the reproductive contribution was clearly reflected by a drop in adult somatic weight, particularly in incubating females. Previous data on abundance for this population suggested inter-annual differences of a high magnitude, with periods (e.g. 1986–1988) with abundances reaching 2–6× that presently reported. Probably, stable environmental conditions (e.g. 1986–1988) promote high abundance and survival of reproducers allowing a large contribution of recruits, which in turn show high degrees of survival through the adult phase. The years reported here (2003–2005), however, represent a period of population decline to lower, perhaps minimum, levels.