During the years 1972--1976 a total of 14.692 ticks of the species Ixodes ricinus (L.), (2.738 adults and 11.954 nymphs) was collected. They were taken from five areas where human Central European encephalitis virus (CEE) infections had occurred. By means of, intracerebral (IC) and subcutaneous (SC) isolation experiments with suckling mice 19 agents, exhibiting the pathogenic characteristics of CEE virus, were isolated from these ticks. In neutralization tests with suckling mice these isolates could be identified as CEE virus strains. From the 14.692 ticks at least 1.29% were virus carriers as evidenced by the number of isolated virus strains. By the same reasoning at least 3.6% of the adults and 0.76% of the nymphs must have contained virus. We were successful in each of the five areas, rather distant from each other, to isolate the virus. The favorable ecological conditions for the main vector Ixodes ricinus in the Federal Republic of Germany give rise to the assumption that this medically important tick-borne disease might be permanently present in many areas.