The objective was to evaluate some morphological characteristics of backyard chickens (n= 255) coming from 65 rural production units located in 52 municipalities of 18 States of the Mexican Republic. The statistical model included sex, state, and municipality within state. Sex affected all the response variables (P<0.0001), except massiveness index (P=0.3369). State affected all the response variables (P<0.0001) and municipality was only important for wingspan (P<0.0001), body weight (P<0.0271) and stockiness index (P<0.0267). Male chickens had greater (P<0.0001) body length (4.85 cm more), wingspan (5.66 cm more), breast circumference (2.94 cm more), shank length (1.73 cm more), body weight (0.65 kg more) and massiveness index (0.99 percentage points) than female chickens. Body weight increased 126 g (P<0.01) in males and 61 g in females (P<0.01) for each centimeter increment in breast circumference. Body weight showed to be moderately correlated (P<0.01) with body length (r= 0.65), wing span (r= 0.49), length of the shank (r= 0.67) and highly correlated with breast circumference (r= 0.76 in males). The Mexican backyard chicken presented significant morphological differences between males and females for traits studied, except for massiveness index.