Measurements with a towed thermistor chain have been used to map temperature fine-structure variability along vertical sections through a frontal shear zone in the seasonal thermocline of the Sargasso Sea. Fine-structure levels with 1--3 m horizontal wavelengths were normalized with local values of the vertical fine-scale stratification to give values of normalized fine-structure variance. Patches of increased levels of normalized variance were found to occur throughout the thermocline but were most extensive in the frontal zone where Richardson numbers (7-m by 450-m averages) were lowest (<1). The largest patches were 5--10 m in vertical extent and horizontally coherent over distances of several kilometers. Away from the frontal zone, patches tended to be smaller and were fewer in number. Simultaneous microstructure measurements along two depths were spatially correlated with the fine-structure patches, implying that many patches were areas of increased mixing intensity. No consistent relationship was found between the patches and features in the vertical stratification, such as inversion zones or sheets and layers. Internal wave overturns on the scale of a few meters high by 10 m in the horizontal were found within the larger patches, suggesting that wave breaking, perhaps enhanced by the background shear, may be an ingredient in the formation or maintenance of some patches.