AbstractResults of a mark‐release study ofRattus sordidus colletti(Gould) on sub‐coastal, treeless plains in the monsoonal north of the Northern Territory of Australia are given for 5 years.R. s. collettiis the dominant component of the small mammal fauna of these plains, with only small numbers of Melomys spp. and Planigale maculata also occurring. Two classes of peak densities were observed. Localized peaks resulted from non‐breeding adultR. s. collettirefuging onto the marginally higher levees during flooding of the plains, and also from refuging into a lower‐lying area at the end of a dry‐season drought. Generalized peak densities resulted from peaks in reproductive effort. In 1972 and 1974, reproduction was confined to the period immediately following the monsoons, but in 1973 breeding continued throughout the dry season, following unseasonal rain in June. Thus, generalized peak densities were observed in one year at the beginning of the dry season, and in another at its end. The relatively shallower flooding and mildness of the 1972–3 wet season resulted in commencement of breeding earlier in 1973 than in 1974, while rapid severe flooding of the plains in December 1974 resulted in mass mortality and failure of the refuging populations of adults to recolonize the lower plains in 1975.R. s. collettiremained extremely rare until November 1976, when the study was terminated.The good conditions which allowed reproduction during the dry season of 1973 were reflected in rapid growth rates at that time, compared with rates approaching zero in the 1974 dry season.R. s. collettiis the most fecund form of Australian Rattus species. This high fecundity appears to compensate for the restriction, by dry‐season aridity and wet‐season flooding, of the breeding season in most years to a short period immediately following the monsoons. In unusual years with rainfall during the dry season, the high fecundity givesR. s. collettipopulations the capacity to reach very