The genus Sphaerocetum Fikáček, 2010 is reviewed on the basis of specimens collected from mixed ant nests shared by Camponotus Mayr, 1861 and Crematogaster Lund, 1831 ants in Peninsular Malaysia. Two new species, S. arboreum, sp. nov. and S. hortulanum, sp. nov., are described. A larva of S. arboreum was collected in the same nest as the adults, implying that it is likely that the entire life cycle takes place inside the ant nest; its association with adults was confirmed by cox1 sequences. It is described in detail and represents the first known larva of the tribe Protosternini. Fragments of four genes (cox1, cox2, 18S and 28S) were amplified for S. arboreum and combined with previously generated data in order to test the position of the genus within the subfamily Sphaeridiinae. The analyses revealed Sphaerocetum as a sister taxon to Protosternum Sharp, 1890, corroborating the monophyly of the tribe Protosternini. Bayesian analysis revealed an alternative hypothesis of the phylogenetic position of the tribe, indicating that Protosternini is a sister-group to Omicrini. This position is supported by the chaetotaxy of the maxillary stipes of the larva, which lacks the increased number of stout setae on the inner face present in all other Sphaeridiinae larvae except Omicrini.