Specimens of Mesochaetopterus (Chaetopteridae) from seven geographical sources in the Pacific Ocean are compared on the basis of their hard structures: specialized chaetae, uncinal plates and tubes. There geographical variations are investigated both locally (New South Wales and the Solomon Islands) and over the whole Pacific Ocean from Australia (New South Wales) through the Solomon Islands to Galapagos and Hawaii. The most interesting result is the existence of intra-regional morphological variations with the hard structures differing on specimens sampled in two areas from New South Wales or in two areas from the Solomon Islands out of a total of three areas. These newly described morphologies imply that M. minutus, isolated in a first step from M. sagittarius and confined to the Pacific, is a pseudo-sibling species complex. Each element of this complex is morphologically distinguishable. Consequently the generally accepted role of the long-lived planktonic larvae characteristic of Mesochaetopterus, as a source of geographical homogeneity must be re-examined.