Two bacterial strains (SC-089^T and SC-092^T) isolated from sewage sludge compost were characterized by using a polyphasic approach. The isolates were Gram-negative short rods, catalase- and oxidase-positive, and showed good growth at 30 °C, at pH 7 and with 1 % (w/v) NaCl. Ubiquinone 8 was the major respiratory quinone, and phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol were amongst the major polar lipids. On the basis of 16S rRNA gene sequence analysis, the strains were observed to be members of the family Alcaligenaceae, but could not be identified as members of any validly described genus. The low levels of 16S rRNA gene sequence similarity to other recognized taxa, together with comparative analysis of phenotypic traits and chemotaxonomic markers, supported the proposal of a new genus within the family Alcaligenaceae, for which the name Candidimonas gen. nov. is proposed. Strains SC-089^T and SC-092^T, which shared 99.1 % 16S rRNA gene sequence similarity, could be differentiated at the phenotypic level, and DNA–DNA hybridization results supported their identification as representing distinct species. The names proposed for these novel species are Candidimonas nitroreducens sp. nov. (type strain, SC-089^T = LMG 24812^T = CCUG 55806^T) and Candidimonas humi sp. nov. (type strain, SC-092^T = LMG 24813^T = CCUG 55807^T).