During the summer of 1964, the city of Houston experienced a major epidemic of St. Louis encephalitis (SLE), the first recorded appearance of arbovirus encephalitis in the city.1 More than 700 clinical cases and 32 deaths were suspected of being caused by SLE. The first recorded epidemic of this disease occurred in St. Louis, Mo, during the summer of 1933.2 Five hundred and twenty cases in St. Louis county and 577 cases in St. Louis city were reported, based entirely on clinical findings. A clinically similar, but smaller, outbreak had occurred in Paris, Ill, in the summer of 1932. The observations made some 30 years age indicated that the disease had no predilection for sex or race but showed a striking increase in both incidence and fatality rates with increasing age. It is well known also that during outbreaks of St. Louis virus encephalitis the inapparent infection rate