Scale patterns, maturational status and otolith microchemistry (strontium to calcium ratios) were analysed in sympatric anadromous and non-anadromous rainbow trout Oncorhynchus mykiss in the Santa Cruz River (Patagonia, Argentina) to investigate the life-history differences of anadromous and non-anadromous lifestyles and the association between maternal origin and progeny life history. The analyses revealed that both forms can give rise to one another, indicating a single population with alternative phenotypes. Anadromous fish smolted at ages 2 and 3 years, matured after 1 to 2 years in the ocean, and survived up to 11 years, spawning up to eight times. Non-anadromous fish survived up to 6 years, spawning up to three times. The extended reproductive life span associated with anadromy in this river suggests that increased energetic and physiological demands associated with ocean migration may not necessarily result in reduced postspawning survival, as has been suggested for salmonids in general. Alternatively, reduction in parity may be regarded as the evolutionary outcome of reproductive traits resulting from the adoption of anadromy (i.e. augmented reproductive investment) coupled with long-range migrations to and from the ocean. The life-history patterns of Santa Cruz River rainbow trout provide a natural experiment for investigating the evolutionary transition and maintenance of anadromy and non-anadromy within salmonid populations.