We have used ground-penetrating radar (GPR) to observe englacial structural control upon the development of an esker formed during a high-magnitude outburst flood (jökulhlaup). The surge-type Skeiðarárjökull, an outlet glacier of the Vatnajökull ice cap, Iceland, is a frequent source of jökulhlaups. The rising-stage waters of the November 1996 jökulhlaup travelled through a dense network of interconnected fractures that perforated the margin of the glacier. Subsequent discharge focused upon a small number of conduit outlets. Recent ice-marginal retreat has exposed a large englacial esker associated with one of these outlets. We investigated structural controls on esker genesis in April 2006, by collecting >2.5 km of GPR profiles on the glacier surface up-glacier of where the esker ridge has been exposed by meltout. In lines closest to the exposed esker ridge, we interpret areas of englacial horizons up to ∼30 m wide and ∼10-15 m high as an up-glacier continuation of the esker sediments. High-amplitude, dipping horizons define the base of esker materials across many lines. Similar dipping surfaces deeper in the profiles suggest that: (1) the dipping surfaces beneath the esker are englacial tephera bands; (2) floodwaters were initially discharged along structurally controlled englacial surfaces (tephra bands); (3) the rapid increase in discharge resulted in hydrofracturing; (4) establishment of preferential flow paths resulted in conduit development along the tephra bands due to localized excavation of surrounding glacier ice; and (5) sedimentation took place within the new accommodation space to form the englacial structure melting out to produce the esker.