We present isotopic analysis of helium in hydrothermal fluids from the Manus back-arc basin, collected during the French-Japanese Manusflux and Manaute cruises in 1995 and 2000, respectively. The helium isotope composition of the fluids is comparable to that measured elsewhere in similar subduction environments, with He-3/He-4 values (7.4 to 7.9 R-a) slightly lower than typical MORB values, except on the Manus Spreading Center (MSC) where higher values (12 +/- 0.1 R-a) are observed. The causal link between the narrow spatial extent of the He-3/He-4 anomaly at the MSC and a He-3-rich deep mantle component remains elusive, based on available geophysical data. Thus, the Manus Spreading Center results may be an interesting example of small scale upper-mantle heterogeneity.