To elucidate environmental parameters controlling stalagmite growth in tropical areas, we conducted cave monitoring throughout a year in Petruk Cave, central Java, Indonesia. We compared stalagmite growth rate with the cave's environmental parameters, air temperature, drip rate, calcium concentration of the drip waters, and pCO 2-air. We found a relationship where stalagmite growth rate is fast (slow) when pCO 2-air is low (high) during dry (rainy) season, suggesting that pCO 2-air controls stalagmite growth. Note that this is a first study that reports that dramatic pCO 2-air reduction occurring during dry season in a tropical cave controls stalagmite growth. Additionally, we discuss the mechanism of pCO 2-air fluctuation. Monitoring results show that pCO 2-air fluctuation is divided into two phenomena: seasonal fluctuations and daily fluctuations. Dramatic pCO 2-air reduction during the dry season is likely to result from a decline of plant activity due to little rainfall. On the daily scale, pCO 2-air reached to the minimum around 6 a.m. and maximum around 2-4 p.m., although it is not obvious whether this is due to plant activity or cave ventilation. Also, dynamic pCO 2air reduction was observed following cave ventilation driven by the sudden drop of outside air temperature due to a downburst during severe rain. This suggests that heavy rainfall in short duration is also one factor that controls cave ventilation and pCO 2-air. introduction Stalagmite geochemistry has been widely recognized as a useful proxy to reconstruct paleoclimate or paleoenvironment of terrestrial areas (e.g., Wang et al., 2001; Fairchild et al., 2006; Wang et al., 2008). If stalagmite growth rate has a seasonal variation, the geochemical signatures might be influenced as well by the seasonal variation in growth rate. Meanwhile, some previous studies have utilized stalagmite growth-rate itself as a paleoclimate or environment proxy (e.g., Proctor et al., 2000; regional precipitation reflecting the strength of the winter North Atlantic Oscillation; Polyak and Asmerom, 2001, wet/dry condition). It is, therefore, important to elucidate the mechanism of stalagmite growth for reconstructing paleoclimate or environment using stalagmites. Recently, cave monitoring studies were conducted to understand the relationships between surface climate and stalagmite characteristics (e.g.,