The polyplacophoran Chiton articulatus Sowerby, 1832, is distributed along the tropical Pacific coast of Mexico and has been collected for generations in an artisanal manner for bait or as a complement in domestic cookery of coastal inhabitants; nevertheless, recently, some restaurants are offering this chiton to the tourists as a gourmet and aphrodisiac food, thus becoming of potential economic interest. This study explores in a population of C. articulatus adults from Oaxaca, southern Mexican Pacific, the changes in morphometry and in somatic and reproductive indices throughout one year, reporting on five reproductive indices, five somatic indices, body shape (BS), and length-weight relationships. An attempt is made to elucidate the energetic trade-off between somatic and reproductive tissues, including an assessment of whether these can be estimated from noninvasive indices. Somatic indices diminished considerably as gonad development proceeded, thus showing an inverse relationship with reproductive indices. Body mass index (BMI, i. e., physical conditions) patterns revealed that chitons were overweight (in the sense of high accumulation of lipids in the gonad) during reproductive seasons. The C. articulatus body was mainly oval-shaped throughout the whole sampling period. The slope of the length-weight regressions indicated an allometric growth (b, between 2.2 and 2.8). Gonad development is fueled by energy and nutrients derived from food rather than from reallocation of somatic reserves, although some somatic tissue constituents (in particular from foot) of reproducing adults may be mobilized in the short-term to meet nutrient and energy needs for reproduction. Reproductive seasons of C. articulatus according to the measured indices, occurred around May (late-spring) and again around August-September (late-summer), and these periods were correlated with maximum sea surface temperatures. The measurement of BS together with BMI can be used nearly as effectively as the more invasive reproductive indices to estimate reproductive seasonality.