AbstractThe heart of the least shrew,Cryptotis parvaSay, is an extremely active organ, capable of achieving rates of 800—1,200 beats/minute. The general features of myocardial cell ultrastructure in this insectivore are much like those of other small mammals; no single striking feature of fine structure is present to which the physiological properties of this heart might necessarily be attributed. Still there exist in these myocardial cells a number of atypical properties. These include (1) mitochondria having a wide variety of sizes and internal configurations (2) a pleiomorphic, highly ramified, small‐diameter transverse‐axial tubular system (TATS) (3) numerous “labyrinths,” which are proliferated components of the TATS, and (4) myofibril‐free regions, located both in juxtanuclear and other myoplasmic levels and populated by a concentration of TATS elements and fibrillar structures. Features (2) and (3) are also characteristic of another fast‐beating heart, that of the mouse. The sinoatrial and atrioventricular nodal regions, as well as a Purkinje system, have been identified in the least shrew heart, along with sparsely distributed atrial cells whose myofibrils contain proliferated Z‐band material. A feature frequently encountered in atrial working muscle cells is the occurrence of close appositions between gap junctions and tubules of sarcoplasmic reticulum; such appositions are also present in other regions of the shrew heart, as are complexes composed of gap junctions a