Biofilms in marine and fluvial environments can comprise strong bacterial and diatom mats covering large areas of the bed and act to bind sediments. In this case the bed material becomes highly resistant to shear stresses applied by the overlying fluid motion and detachment, when it does occur, is manifest in patches of biofilm of the order cm2 being entrained into the flow. This paper is the first to report tensile test data specific to the centimeter scale using moist biofilm/sediment composite materials; the strain (e) - stress (s) relationships permit quantification of the elasticity (Young’s modulus, E) and cohesive strength of each specimen. Specifically, we compare the mechanical strength of cyanobacterial biofilm-only samples to that of biofilm cultured over sediment samples (glass beads or natural sands of d~1mm) for up to 8 weeks. The range of tensile strength (1,288-3,283Pa) for composite materials was up to 3 times higher than previous tensile tests conducted at smaller micrometer scales on mixed culture biofilm (Ohashi et al., 1999), yet of similar range to cohesive strength values recorded on return activated sludge flocs (RAS; Poppele and Hozalski, 2003). Composite materials were 3-6 times weaker than biofilm-only samples, indicating that adhesion to sediment grains is weaker than cohesion within the biofilm. Furthermore, in order to relate the tensile test results to the more common in-situ failure of bio-mats due to shear flow, controlled erosion experiments were conducted in a hydraulic flume with live fluid flow. Here, the fluid shear stress causing erosion was 3 orders of magnitude lower than tensile stress; this highlights both the problem of interpreting material properties measured ex-situ and the need for a better mechanistic model of bio-mat detachment.