Abstract: An ultrasonic transducer is improved, particularly for use in perivascular blood flow measurement, by the provision of a coupling member made of a hydrogel. The hydrogel is suitably acrylic based; has a water content in the range from 75 to 90% by weight and preferably about 85%; and has acoustic attenuation of less than 2 dBcm.sup.-1 at 5 MHz, with longitudinal velocity of sound and acoustic impedance of similar order to those for tissue. The crystals of the transducer are preferably sealed from the coupling member by a water resistant coating, the member suitably has an exposed face of part cylindrical shape to accommodate a vascular vessel, and the member is preferably releasably mounted to allow size interchange for different vessels.