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Abstract The large‐scale expansion of Moso bamboo forests in Japan raises concerns about how vegetation will affect the hydrological cycle and local water resources. However, information on transpiration ( E ) is limited to study on a single bamboo stand with low culm density (4 000 ha −1 ). The present study conducted a year‐round measurement of E in a high‐density (11 000 ha −1 ) stand using sap‐flow measurements, and additional measurements were made during summer in the previously studied low‐density stand and a neighbouring Japanese cedar stand for comparison. Annual E of the high‐density stand was estimated at 455 mm. Although the value was greater than those reported for neighbouring coniferous forests (250–370 mm), it was lower than previous measurements made in the low‐density stand (567 mm). The daytime sap flux density of the high‐density stand was generally less than half that in the low‐density stand, resulting in similar daily maximum E values in those stands. In addition, E typically peaked before midday in the high‐density stand followed by a rapid decline; the midday suppression of E led to decreased total E in the stand. Based on present and previous measurements of E , combined with the lower rainfall interception ( I ) reported for bamboo stands, water loss through evapotranspiration would be comparable between bamboo and coniferous forests. Present results also suggest that the E of bamboo stands does not have strong positive relationships with stand structural parameters (e.g., culm density and sapwood area) but possibly depends more on the sites' microenvironments (e.g., solar radiation). Copyright © 2014 John Wiley & Sons, Ltd.