The active and passive contractile performance of skeletal-muscle fibers largely depends on the myosin-heavy-chain (MHC) isoform and the stiffness of the titin spring, respectively. Here, a large set of adult rabbit muscles (n=37) was studied for titin-size diversity, passive mechanics, and possible correlations with the fiber/MHC composition. Titin-isoform analyses showed sizes between ~3300 and 3700kDa; 31 muscles contained a single isoform, six muscles coexpressed two isoforms, even at the single-fiber level. Fiber and single-myofibril mechanics revealed an inverse relationship between titin size and titin-based passive tension. Force measurements on muscle strips suggested that the relative contribution of titin to total passive stiffness varies greatly among muscles, as extramyofibrillar structures also contribute. Plots of titin size versus percentage of fiber type or MHC isoform (I-IIB-IIA-IID) determined by myofibrillar ATPase staining and gel electrophoresis revealed modest correlations with the type-I-fiber and MHC-I proportions. Titin-based stiffness decreased with the slow-fiber/MHC percentage. However, no relationships were found with the proportions of the different type-II-fiber/MHC-II subtypes. Thus, the active and passive mechanical properties of muscle fibers show a low correlation. Slow muscles usually express long titin(s), predominantly fast muscles can express either short or long titin(s), giving rise to low titin-based stiffness in slow muscles and highly variable stiffness in fast muscles.