Nutrient utilisation and growth performance responses of White Pekin ducks (Anas platyrinchos domesticus) offered diets containing low- or high-viscosity wheat supplemented with xylanase were investigated in two studies. In Expt 1, six diets consisting of low-viscosity wheat or high-viscosity wheat supplemented with 0·0, 1·5 or 3·0 g xylanase (2590 units/g)/kg diet were used in a true metabolisable energy (TME) bioassay with eight 8-week-old ducks per diet group. In Expt 2, eight pens of ten 3-d-old ducks per pen for each of six wheat-based diets arranged in a 2 × 3 factorial of low-viscosity or high-viscosity wheat and 0·0, 1·5 or 3·0 g xylanase/kg were used in a 42 d growth study. High-viscosity wheat depressed (P < 0·001) TME and xylanase supplementation improved (P < 0·001) TME, more so for high-viscosity than low-viscosity wheat. Xylanase supplementation of the high-viscosity wheat-based diet improved (P < 0·05) weight gain and gain:feed ratio by 13 and 12 % respectively. There was no weight gain or gain:feed ratio response to xylanase supplementation of the low-viscosity wheat-based diet. Xylanase supplementation reduced (P < 0·001) the viscosity of duodenal and ileal digesta for high-viscosity but not low-viscosity wheat-based diets. Ileal digestibilities of nutrients and energy were higher (P < 0·001) for low-viscosity than high-viscosity wheat-based diets; xylanase supplementation improved (P < 0·05) energy, fat, N and starch digestibilities. Given that xylanase supplementation of high-viscosity wheat assuaged its anti-nutritional effect, it is surmised that digesta viscosity plays a role in anti-nutritional effects in wheat-based diets for ducks.