Two known and two new species of the griseifrons species-group of the subgenus Simulium (Simulium) in Northern Thailand are reported. The male, pupa and mature larva of S. (S.) rudnicki Takaoka and Davies, and the pupa and mature larva of S. (S.) suchariti Takaoka and Choochote, are described for the first time. Furthermore, two new simuliid species, S. (S.) mediocoloratum sp. nov. and S. (S.) crocinum sp. nov., both of which are very similar to S. (S.) rudnicki, are also described.