We report observations of transient ionospheric disturbances at VLF reflection heights, manifested as perturbations on sub-ionospherically propagating VLF transmitter signals: ''Trimpi events.'' Four VLF transmitter signals were monitored at Durban using an OMSKI receiver. Both traditional Trimpi events and a new ''dome'' shaped event were observed at Durban. Occurrence of Trimpis at low L values can be interpreted as the precipitation of very high energy electrons which are capable of producing events during daylight, or as evidence of the ''fast'' Trimpi mechanism which could dominate at low L. The average winter event rate observed at Durban was quite low (7 per day on four transmitter signals) but during one very active period on the NAA-Durban signal 88 Trimpi events were observed in 2 hours. This activity was linked to an isolated thunderstorm occurring over the signal path over South Africa. ¿American Geophysical Union 1993