This paper describes the expressed models that children aged 7, 11, and 14 years have about micro-organisms and microbial activity. These were elicited using a variety of data collection techniques that complemented each other, resulting in a rich dataset, and provided information about the level of knowledge and progression of ideas across the age range studied. Subsequent analysis of the data enabled generalised mental models of micro-organisms to be derived that indicate a hierarchy in the sophistication and accuracy of the concepts explored. The resulting mental models were categorised as extended, transitional, and emergent and characterise the ideas held by some children in each of the age groups studied. The implications for curriculum development and consequences for learning and teaching microbiology are discussed. (Contains 1 figure and 12 tables.)