The rate and extent of crystallization in crosslinked samples of natural rubber (NR), cis-1,4-polybutadiene (BR), and butyl rubber (IIR), were studied by observing the relaxation of stress in stretched strips, held at low temperatures. Melting temperatures were measured from the recovery of stress on warming. The melting temperature was raised by stretching, and the rise was significantly greater for NR than for BR, consistent with the lower heat of fusion of NR. In some cases crystallization was also followed by volume changes, or by DSC or x-ray diffraction. The maximum degree of crystallization was estimated to be only about 20% for BR, 28% for NR, and somewhat higher for IIR. On raising the temperature the tensile strength showed a marked drop to only 1–2 MPa when the elastomer failed to crystallize on stretching. At lower temperatures, when strain-induced crystallization occurred, the tensile strength was much higher, but the values were different for the three elastomers: about 10 MPa for BR, 20 MPa for NR, and 30 MPa for IIR, roughly in proportion to the inferred extent of crystallization. We speculate that molecular entanglements (more dense in BR and less dense in IIR than in NR) are severe obstacles to crystallization in high-molecular-weight polymers. Some specific molecular features are inferred that are needed for an elastomer to crystallize on stretching and to exhibit high strength over a wide temperature range.