On the basis of physical and biochemical tests plus host range studies a wilt disease of potatoes in Colombia, known as “la dormidera,” has been determined to be caused by the bacteriumPseudomonas solanacearum E.F.S. Colombian isolates of the organism differ from those described in other regions with respect to reaction to antibiotics and other therapeutic materials, optimum temperature for growthin vitro and ability to cause disease at low temperatures. Colombian isolates were able to cause disease in the field and in greenhouse and growth cabinet inoculation tests at temperatures lower than those previously recorded.