--- - |2+ Abstract Aim Constitutional thinness (CT) is a rare condition of natural low body weight, with no psychological issues, no marker of undernutrition and a resistance to weight gain. This study evaluated the skeletal muscle phenotype of CT women by comparison with a normal BMI control group. Methods Ten CT women (BMI < 17.5 kg/m2) and 10 female controls (BMI: 18.5‐25 kg/m2) underwent metabolic and hormonal assessment along with muscle biopsies to analyse the skeletal muscular fibres pattern, capillarity, enzymes activities and transcriptomics. Results Constitutional thinness displayed similar energy balance metabolic and hormonal profile to controls. Constitutional thinness presented with lower mean area of all the skeletal muscular fibres (−24%, P = .01) and percentage of slow‐twitch type I fibres (−25%, P = .02, respectively). Significant downregulation of the mRNA expression of several mitochondrial‐related genes and triglycerides metabolism was found along with low cytochrome c oxidase (COX) activity and capillary network in type I fibres. Pre‐ and post‐mitochondrial respiratory chain enzymes levels were found similar to controls. Transcriptomics also revealed downregulation of cytoskeletal‐related genes. Conclusion Diminished type I fibres, decreased mitochondrial and metabolic activity suggested by these results are discordant with normal resting metabolic rate of CT subjects. Downregulated genes related to cytoskeletal proteins and myocyte differentiation could account for CT's resistance to weight gain. - Acta Physiologica, Volume 224, Issue 3, November 2018.