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We studied age, growth, and sexual development in the early life intervals of the self-fertilizing mangrove killi-fish, Rivulus marmoratus. Newly hatched (day 0) individuals had sagittal otoliths of 60 μm radius, with about 30 increments. Sequential sampling until about day 60 after hatching yielded otoliths with the number of increments outside the 60 μm radius equal to the daily age of the fish. Alizarin complexone marking of otoliths also confirmed the increments were daily, and demonstrated the applicability of this technique to field studies for capture-mark-recapture, or age and growth estimates. Individuals fed a restricted amount of food formed fewer daily otolith growth increments than fish fed to satiation each day. Using histological analysis for identifying gonad morphogenesis, we found no correlation between gonadal development and external appearance (caudal ocellus, orange fin colouration) in young fish of known ages. The caudal ocellus was not present until 9 mm total length, and developed thereafter. Of 136 individuals examined, fish less than 17.2 mm total length (TL, n = 124) were females. Testicular tissue first appeared among individuals 17–18 mm TL (n = 3), while some individuals greater than 18 mm TL (n = 8) were functional hermaphrodites. The single male in our study was relatively of small body size (9.6 mm, day 37) with a distinct caudal ocellus, indicating that it is presumably a primary male.