The authors followed, during 4 years consecutively, from 1987 to 1990, by immunofluorescence, the frequency of B. burgdorferi in an amount of 677 nymphs of I. ricinus tick, collected fasting by flagging in 2 forests in Brittany (France). Percentages obtained in each of these forests do not reveal significative differencies statistically and seem to show a relative stability, from one year to the following, during the considered period, of the infestation levels in ticks.