The new Simrad scientific multibeam systems, the MS70 sonar and the ME70 echosounder, each transmit over many electronically formed beams with centre frequencies spanning from 70 to 120 kHz. Calibrations of these systems are therefore more complex than for conventional split-beam echosounder systems. Two large tungsten-carbide spheres (75 and 84 mm diameter) were designed and manufactured to facilitate accurate field calibrations over the entire operational bandwidth. These are heavy and therefore stable when suspended beneath a ship, and have target strengths much larger than those of biological targets potentially within the measurement volume. This paper presents procedures for calibrating each system in the field and the results from two such experiments. Detailed inspections of the results for individual beams indicate that minor adjustments in the described procedures might further improve the reported calibration accuracy.