This study assessed Zambian Junior High School pre-service science teachers' understanding of the particulate nature of matter. A sample comprised 30 pre-service science teachers at a teacher training college. Data was collected through a questionnaire adopted from Ozmen and Kenan (2007). Results show that most teachers had correct views on the effect of phase change on speed, spaces and number of particles in a substance. However, most pre-service teachers had poor understanding about the effect of phase change, cooling and heating on the size of the particles in a substance. Most pre-service teachers erroneously believed that when a substance is cooled it contracts because the sizes of its particles decrease, and when a substance is melting the size of its particles increase. Results have implications on science teaching and learning and teacher education. (Contains 3 tables.)