The Hennessy and Chong Fat Depth Indicator (FDI) and a compact, ultrasonic instrument (Renco) were compared using 152 pork carcasses. Fat thickness measurements were taken 5 cm off the midline at five locations (shoulder, SF; mid-back, MB; last rib, LR; loin, LO; and lumbar, LU) on both sides of the carcass immediately after sticking (before scalding), after splitting the carcass and 24 h postslaughter (FDI only). Actual thicknesses were determined, using a ruler, for these locations on the left side and at the midline of the split carcass. Instrument readings made on the right or left side of the carcass were not different (P > 0.05). Precision (similarity of first and repeated reading of fat thickness) was higher for the FDI than the Renco with average residual standard deviations for both instruments being 1.2 mm and 1.4 mm, respectively. Precision was similar for all locations except the SF site which had significantly (P < 0.05) poorer values. The average bias was smaller for the FDI than for Renco, but was not large for either instrument except at the shoulder. However, regressions of instrument readings on ruler measurements for both instruments gave slopes that were considerably lower (P < 0.05) than unity with positive values for intercepts. This indicates that fat thickness would be over- and under predicted in lean and fat carcasses, respectively, using both instruments. Key words: Swine, carcass backfat, ultrasonic, fat probes