Concept mapping is a technique used to provide a visual representation of an individual's ideas about a concept or set of related concepts. This paper describes a concept mapping tool using a photograph association technique (CoMPAT) that is considered to be a novel way of eliciting children's ideas. What children at 11 years of age know about particular concepts related to microbial activity is presented and discussed in relation to the effectiveness of CoMPAT as a tool to draw out their ideas. It is proposed that this tool could be used to explore ideas about this and other science concepts from children in other age groups, and where language may be a barrier to participation. (Contains 4 tables and 6 figures.)