Studies about gender difference in mental rotation ability are well documented in the context of developed countries; though not in developing or low-income ones. This paper examined students' mental rotation ability as a function of gender in a disadvantaged community in Indonesia. The Spatial Reasoning Instrument (SRI) was used to test 334 students (aged 13-14 years old) in an untimed test. The characteristics of the SRI items were analysed in relation to the gender difference. The results showed a significant difference in favour of males on both 2D and 3D mental rotation tasks. The effect of the task direction was mediated by gender; girls performed significantly lower than boys for the task with an implicit direction.