'Caulobacter leidyi' DSM 4733(T) has been shown to be affiliated with the family Sphingomonadaceae instead of the Caulobacteraceae, and due to its poor characterization has been omitted from the current edition of Bergey's Manual of Systematic Bacteriology and removed to limbo. We isolated a novel sphingoglycolipid-containing dimorphic prosthecate bacterium, designated strain 247, from a pre-alpine freshwater lake. Strain 247 and 'Caulobacter leidyi' DSM 4733(T) were characterized in detail. The rod-shaped cells were Gram-stain-negative, aerobic, catalase- and oxidase-positive, and formed a stalk or polar flagellum. Both strains grew optimally at 28-30 °C, and pH 6.0-8.0. The major fatty acids were C(18 : 1)ω7c, C(16 : 0) and 11-methyl C(18 : 1)ω7c. C(14 : 0) 2-OH represents the major 2-hydroxy fatty acid. Q-10 was the major respiratory quinone and the major polar lipids were diphosphatidylglycerol, phosphatidyldimethylethanolamine, phosphatidylglycerol, phosphatidylmonomethylethanolamine, phosphatidylethanolamine, phosphatidylcholine, three glycolipids, two phosphoaminolipids and two unidentified sphingoglycolipids. The major polyamine was sym-homospermidine. The G+C content of genomic DNA of strains 247 and DSM 4733(T) was 67.6 mol% and 67.0 mol%, respectively. According to 16S rRNA gene sequence analysis and DNA-DNA hybridization, strains DSM 4733(T) and 247 were phylogenetically closely related (99.6 % 16S rRNA gene sequence similarity, 82.9 % DNA-DNA hybridization value) and affiliated to the genus Sphingomonas. The closest recognized species was Sphingomonas aquatilis DSM 15581(T) (98.1 % sequence similarity). In addition, the presence of cystine arylamidase, absence of β-galactosidase, and the inability to utilize l-arabinose, galactose and sucrose distinguished strains DSM 4733(T) and 247 from most other members of the family Sphingomonadaceae. So far, the dimorphic life cycle that involves a prosthecate and a flagellated stage is unique for strains DSM 4733(T) and 247 among all members of the family Sphingomonadaceae. Therefore, Caulobacter leidyi is reclassified as Sphingomonas leidyi, with the type strain DSM 4733(T) ( = ATCC 15260(T) = CIP 106443(T) = VKM B-1368(T)) and strain 247 (DSM 25078 = LMG 26658) as an additional strain of this species.