The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence F-th similar to 0.4 J cm(-2) and the volume ablation rate was measured to be 30 < V < 450 mum(3) per pulse in the fluence range 1.4 < F < 21 J cm(-2). The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F > 7 J cm(-2) and >85 W cm(-2) average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F similar to 1.4 J cm(-2) with low power density, similar to3 W cm(-2) produces much superior surface micro-structuring with minimum melt and measured surface roughness R-a similar to 1.1 +/- 10.1 mum at a depth D similar to 50 mum. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm(-2) combined with single pulse fluence <4 J cm(-2) produces near optimum microstructuring. The debris under these conditions contains pure aluminium naroparticles carried with the helium stream. (C) 2004 Elsevier B.V. All rights reserved.