A cladistic analysis of the species of Calloserica Brenske, 1894, was executed using 49 morphological characters of adults. The generated phylogenetic tree supports the monophyly of the genus and the sister-group relationship between Pachyserica and (Gastroserica, Neoserica). Allopatric speciation is discussed as an important mechanism for explaining present tree topology and distribution patterns. Emergence of adults (and mating) during the monsoon season is considered to be a major factor for stimulating the diversification of this group, resulting in its highly restricted distribution and a high degree of endemism. Two new species are described from Nepal: Calloserica capillata, sp. nov. and C. manangensis, sp. nov.