A natural, tidal-flat population of Suaeda japonica was studied to determine how air temperatures early in the growing season affected population formation. Changes in germination rates, density, standing crop (amount of biomass aboveground), and plant heights and weights were monitored from February 1999 to September 2000. Seed germination occurred between mid-February and early May in 1999, and from mid-February to mid-April in 2000. The daily mean air temperature fluctuated below and over $0^{circ}C$ through the end of March in 1999 and until mid-March in 2000. Seedling losses due to freezing were greater in 1999 than in 2000. Densities in 1999 and 2000 were 790 and $2201 plants/m^2$ in the early growing season, and 578 and $803plants/m^2$ later on, respectively. Therefore, the density and mortality of S. japonica was higher in 2000 than in 1999. Values of correlation were low between density and standing crop, but diverse between density and height. The variation in standing crops was remarkable but that of height was small among sites. Frequencies of size classes, based on height, followed normal distribution curves over time, but those based on weight were very great in the lower classes over the entire growing season.