The altitudinal distribution and density of host-seeking Ixodes ricinus ticks were investigated by flagging in 2002Ð2006 along two vertical transects in the Krkono_e Mts. Comparison with historical data revealed that the I. ricinus upper distribution limit shifted from the former 700Ð800 m limit at least up to 1100 m above sea level (a.s.l.) during the last two decades. The tick thus has newly colonised the whole montane belt, covered predominantly with Norway spruce forests. The density of host-seeking nymphs decreased with increasing altitude: In the established mixed forests located at 600Ð800 m a.s.l., it reached the mean value of 31 nymphs per hour of flagging (with a maximum density of 125 nymphs per hour). The mean density attained 7.1 nymphs per hour (maximum 66) in 870Ð1020 m and 2 nymphs per hour (maximum 6.3) in the newly colonised areas at 1080Ð1100 m a.s.l. Regular records of host-seeking I. ricinus larvae and nymphs up to 1020 m a.s.l., together with rather regular records of nymphs and presence of single host-seeking adults up to 1100 m a.s.l. indicated viability of the newly established local tick populations. The present empirical upper limit was experimentally confirmed by the investigation of the developmental capability of eggs, fed larvae and nymphs along a vertical transect (650Ð1550 m a.s.l.) in 2004Ð2006. The experimental results demonstrated a strong relationship between tick developmental success and duration on the one hand and altitude on the other. They showed that climatic conditions up to 1100 m a.s.l. are at present suitable for the development of all tick stages. However, it also revealed that ticks were able to complete their development up to at least 1250 m a.s.l. in the unusually warm summer and autumn 2006 (the mean temperature for JuneÐOctober 2006 exceeded the 1960Ð1990 mean value by 1.8 ¡C). The causes of tick expansion could be found when analysing long-term (1961Ð2005) climatic data from the Krkono_e Mts., which showed a systematic and significant rise of the mean annual air temperature by 1.4 ¡C at 1000 m a.s.l. The most pronounced temperature increase (mean monthly temperature by 1.5Ð3.5 ¡C at 1000 m a.s.l.) was determined in MayÐAugust, which is crucial for host finding and development of I. ricinus. Abundance of the newly settled local tick populations at higher altitudes may increase in the future after their closer establishment in local mountain ecosystems.