An instrument for measuring shear stress for erosion in situ (ISIS) has been developed to measure the erosion shear stress of muddy sediments on intertidal mud flats. Erosion shear stress is defined in this paper as the minimum applied bed shear stress required to initiate erosion and remove sediment from the bed surface. An applied shear stress is generated by the flow through and around a specially shaped bell head, which draws water radially across the bed into the centre of the bell head. The applied shear stress is a function of the distance from the bell head to the bed surface and the discharge through the system. The design of ISIS was assisted by the use of a computational numerical flow modelling package. The operating conditions giving the most even shear stress across the whole test section were discharges of 0.01-0.6 ls(-1), and bell-to-bed distance of 4-8 mm giving a shear stress of 0.02-5 Nm(-2). The ISIS system was calibrated using hot film shear stress probes. The calibration data gave a 92% fit to the calibration function for shear stress. Laboratory measurements with ISIS of the erosion shear stress of mud beds consolidated for c. 1.5 days, showed surface shear stresses of 0.11-0.24 Nm(-2). These were very similar to values of surface erosion shear stress measured for the same mud in an annular flume. The ISIS system was used to measure surface erosion shear stresses on the mud flats at Portishead and Blue Anchor Bay in the Severn Estuary, U.K. Surface erosion shear stresses at Portishead were generally in the range 0.2-0.5 Nm(-2). The surface erosion shear stresses measured at Blue Anchor Bay, which included mud and sand, ranged between 0.1-1.9 Nm(-2). (C) 1996 Academic Press Limited