This work identifies features of the life history of the deep‐living planktonic foraminiferGloborotalia truncatulinoidesthat offer possibilities for reconstructing characteristics of the past ocean's main thermocline. Part 1 considers two aspects of variation in shell size: the predictability of growth‐related size changes, which we need to calculate changes in shell chemistry, and a model for environmentally forced changes in shell size frequency, which suggests how shell size may be a direct source of paleoceanographic information. Even though size variation among shells ofG. truncatulinoidesat the same stage of growth (same number of chambers) exceeds the size difference between successive growth stages, growth‐related size changes are predictable because shells tend to grow at the same rate. Variation in initial shell size is sufficient to account for the large size variation found among shells at the same stage of growth. Several lines of evidence suggest thatG. truncatulinoidesreproduces at ∼600 m in the water column, and it appears that vertical mixing to that depth is required to return juvenile shells to the surface. Anything that prevents this, such as shallowing of the thermocline, can interfere withG. truncatulinoides'life history in such a way that its shell size‐frequency distributions are altered. Among shells falling to the seafloor from a reproducing population are large numbers of both juvenile and adult sizes and, if reproduction is inhibited, the frequencies of those sizes are reduced and the relative frequency of intermediate‐sized shells increases. A paleoceanographically useful result of this is that the topography of the main thermocline tends to be reflected in the size‐frequency distribution ofG. truncatulinoides'shells from the underlying sediments: In areas where the top of the main thermocline is shallower than ∼600 m, production of juveniles (125–177 µm) and reproductive adults (>425 µm) is reduced, leaving predominantly intermediate‐size