Development of Erysiphe graminis germlings attacking living barley tissues, was monitored under humid and and incubation conditions. In arid conditions, attachment of the short, non-appressorial, primary germ tube to the host surface, appeared to be vital to germling survival. Under either humid or and conditions, most primary tubes became attached, but some failed to do so, either naturally or after micromanipulation was used to displace the tube tip. The unattached germlings grew well in humid conditions, but quickly shrivelled and died in arid conditions. The results indicate that germination in arid conditions is a two-stage process. The first depends on water from within the spore and consists of primary germ tube formation and attachment. The second depends on water taken up from the host by the primary germ tube and consists of appressorial germ tube growth. It follows that any host character which limits primary germ tube. attachment will decrease the survival of germling populations under unfavourably dry environments, giving a form of resistance that probably would not be race-specific.