A study of nine species of small free-living amoebae has been made under standardized and reproducible cultural conditions, by a new method that enables specimens in all stages of division to be obtained easily. In all species the resting nucleus shows a Feulgen-negative nucleolus and Feulgen-positive chromatin granules. Nuclear division in these species and in other amoebae described by other workers is of two main types on which it is proposed to create two new families-Schizopyrenidae and Hartmannellidae. In Schizopyrenidae, the type genus Schizopyrenus n.g. and two other genera, Naegleria and Didascalus n.g., are defined. Naegleria gruberi, Didascalus thorntoni n.sp., Schizopyrenus russelli n.sp., S. erythaenusa n.sp. and S. atopus n.sp. are described. In Hartmannellidae the type genus Hartmannella is defined. H. glebae, H. rhysodes n.sp., H. leptocnemus n.sp and H. agricola are described. The relation of the proposed classification to previously defined families and genera of amoebae, and its bearing on phylogeny are discussed.