Dengue 2 and dengue 4 viruses were isolated and re-isolated by inoculation into Aedes pseudoscutellaris continuous cell line (Mos 61) and/or Toxrhynchites brevipalpis. The strain of dengue 2 had been isolated from a patient returning from Casamance (south-western Senegal) and two strains of dengue 4 from patients who lived in Dakar and had not been outside the town in the 15 days before becoming ill. Serological evidence of dengue 4 infection was found in another patient living in Casamance.