THERE is a serious discrepancy between determinations of the column density of interstellar matter in the direction of the Crab Nebula. Virtually every soft X-ray observation1-5 with sufficient energy resolution to derive this column density yields a result substantially in excess of determinations based on 21-cm absorption6-8. The quality of the X-ray and radio data is now such that this disagreement must represent a genuine physical effect rather than an artefact of the data analysis.