Since 2002, the expansion of Ixodes ricinus ticks and tick-borne infection agents have been studied in the Krkono_e Mts., Czech Republic. Tick-borne encephalitis virus was detected by means of RT-PCR. In 2003, it was detected in 2 out of 491 ticks at 620 and 710Ð720 m a.s.l., respectively, and in 3 out of 939 ticks at 600 m a.s.l. at the same locality in 2004. In 2005, tick-borne encephalitis virus was detected in 5 out of 295 ticks at 900Ð1100 m a.s.l., which is above the formerly known altitudinal limit of I. ricinus distribution. The reason for that could be found in the changing climate. Based on the meteorological data collected in the Krkono_e Mts., 1961Ð2005, there was a significant increase in the mean annual temperature (1.3Ð1.4 ¡C) over that period, namely by 2Ð3.5 ¡C in May through August. Thus, with respect to the average vertical temperature gradient in summer of about 0.6 ¡C/100 m, 2 ¡C correspond to 300Ð350 m in altitude, and accordingly 3.5 ¡C correspond to a shift in altitude of approximately 550Ð600 m, that being in accordance with environmental conditions of the former I. ricinus altitudinal limit confirmed in the Krkono_e Mts. 20 years ago.