<p>This study investigates the scaling exponent (<i>b</i>) of standard metabolic rate (SMR) in western mosquitofish, focusing on both among- and within-individual levels while exploring the influences of growth and sex on within-individual <i>b</i> values. For group-reared fish, the intraspecific <i>b</i> value was determined by analyzing the relationship between SMR and body mass across individuals of varying sizes and ages. For individually-reared fish, the SMR of each fish was measured five times between 60 and 150 days of age, and both among- and within-individual <i>b</i> values were calculated. The among- and within-individual <i>b</i> values of the individually-reared fish across ages did not differ significantly from the intraspecific <i>b</i> value of the group-reared fish. The among-individual <i>b</i> values showed a nonsignificant V-shaped change with increasing age in the individually-reared fish, suggesting that the ontogenetic decrease of <i>b</i> value related to growth slowdown can be reversed post-maturation. Although there was no significant difference in within-individual <i>b</i> values between females and males, greater variability was observed among females. Furthermore, the relationship between within-individual <i>b</i> values and specific growth rate displayed sexual dimorphism, with females exhibiting a stronger positive correlation, suggesting higher energy demands associated with both somatic and gonadal development compared to males.</p>