Comment on "direct observation of laser filamentation in high-order harmonic generation".

We argue for a different physical interpretation of the results given in the recent Letter by Painter et al. [Opt. Lett.31, 3471 (2006)] in which an elongated Ti:saphire beam with two distinct waists is considered as direct evidence of laser filamentation. As the pulse power is well below the critical power for self-focusing, the authors pleaded for new examination of the n2 value for He. A three-dimensional numerical modeling, using the published n2 value for He, reproduces very well the measured data and invalidates the filamentation hypothesis.