The collection here described was sent for determination by Dr. K. Kertész to Mr. F. V. Theobald in 1911, and was passed on to the writer by Mr. Theobald in 1911. Most of the specimens were collected by Fiebrig, and these unfortunately bear no exact data. A few (including the type of Janthinosoma paraguayensis) were collected by Vezényi, mostly at Asuncion in 1904. Though not large, the collection contains a high percentage of apparently undescribed forms, which are dealt with below. The holotypes of two of the new species are in the collection of the Budapest Museum, but in those cases where the species has been described from a number of cotypes, the material has been divided between the Budapest Museum and the British Museum. Since little seems to have been recorded concerning the mosquitos of Paraguay, I give first a list of all the species included in the collection.