The results of a field study to determine the shape ratio of Mode 1 fractures formed in sedimentary rocks are presented. Pelites with isolated sandstone layers in the red Permian sandstones oi the Lodeve Basin (Herault, France) were studied. The fracture system in the pelites consists of a set of metre-sized N 110 degrees trending joints mineralized with calcite. Their trace on undulating topography is very clear. The vertical extension of the fractures is sometimes limited by sedimentary discontinuities such as centimetre-thick sandstone or carbonate nodule layers. The shape ratio defined in tabular conditions by the largest vertical over horizontal dimension (H/L) can only occasionally be measured when one of the fracture walls has been removed by erosion. These fractures appear as rough ellipses with a H/L ratio of about 0.5, which seems to represent the maximum ratio for existing fractures. Measurements on oblique traces on the topographical surface yield no information about the shape ratio of individual fractures, so the distributions of the dimensions of the horizontal and vertical traces alone were measured. The ratio of the mean H over the mean L on such traces is 0.53. As this ratio of the means is very close to the maximum ratio of the observed individual fracture plane, the existence of any great proportion of fractures with H/L smaller than 0.5 is unlikely. Thus a high shape ratio is expected for the most represented fractures, independent of the fracture size. This suggests that most of the fractures were propagated as homothetical ellipses with high shape ratio. The thickness of calcite infilled veins, which can be assimilated to apertures before mineralization, was measured on individual fractures. This thickness increases with the height and length of the traces, and for long fractures it tends to remain constant and to be associated with irregular profiles. In isolated sandstone layers in pelites (5 cm to 20 cm thick) fractures are continuous, straight, with very small (less than 0.5 mm) aperture, and their height is typically limited to the thickness of the layer so that these fractures can be assimilated to rectangles with a small H/L ratio (measured down to 0.013). In pelites, some of the sedimentary discontinuities limit the vertical persistence (or extension) of a small proportion of the fractures, whereas other discontinuities are crosscut by the fractures. This relationship, which can have a great influence on the shape ratio, has been measured by a stopping coefficient (Ca) which is the ratio of the number of fractures abutting on one discontinuity with respect to the total number of fractures abutting or crosscutting this discontinuity (0