Null models are an essential tool for investigating structure in natural communities of animals, including reptiles. In this paper, we studied the assembly structure of a lizard community constituted by four species (Lacerta bilineata, Podarcis muralis, P. sicula, Chalcides chalcides) along 25 different transects, each 300 m long and representing a specific habitat type, in five independent urban green areas in Rome, central Italy. Lacerta bilineata was observed in 92% of the total transects (n = 25), P. muralis in 100%, P. sicula in 72%, and C. chalcides in 52%. Based on the number of lizards observed along the various transects, it seemed that each species was linked especially to particular habitat types within each study area, but that the habitat types frequented by each species were not necessarily exactly the same across the study areas. Null model analyses revealed that the lizard community was not randomly organized in four of five study areas by RA2 (thus denoting that the generalist-specialist nature of the species reduced ecological similarity) but not by RA3 algorithms (thus denoting that the types of resources used did not reduce ecological similarity). Thus, the community structure was due mainly to the different specialist-generalist nature of the various co-occurring species.