Previous results suggested that the gastric function plays a paramount role in digestive efficiency differences between D+ and D- broiler lines divergently selected for AME(n) (more feed efficient and less feed efficient, respectively). In this paper we show an histological analysis of the gastric isthmus located between the proventriculus and the gizzard in the D+ and D- lines. Cross-sections were performed using a cryostat (Leica CM30505) and stained with a routine procedure using Mayer's Hematoxylin and Eosin Stain. The surface and shape of the constitutive gastric isthmus tissues were quantified using the image analysis software Image J. The lumen occupied 11% of the whole D- isthmus cross-sectional area against 24% for D+ (P < 0.01). The mucosa relative area (cm(2)/total cross-sectional area) was higher in D- than in D+ [ 47% (D-) and 39% (D+), P < 0.01]. It was significantly more oval and more folded on the lumen side in D- than in D+ chickens; the muscle layer (muscularis mucosae) of the mucosa was relatively more developed in D- than in D+ (16 and 11% of the section, respectively; P < 0.01). A relationship between these observations and increased gastric motility reported in D- compared with D+ is discussed.