This paper describes a precise new method for determining finite longitudinal strains in porphyroblastic metamorphic rocks, which makes use of oppositely-concave microfolds (OCMs) formed by heterogeneous strain of the matrix around porphyroblasts. The initial spacing between two foliation surfaces is measured inside a porphyroblast and compared to the spacing between the same two surfaces in the matrix, which results in a measure of extension (e) experienced by the rock during and/or after porphyroblast nucleation. A natural example is provided by the well-known ‘millipede’ plagioclase porphyroblasts from the Robertson River Metamorphics in Queensland, Australia. Twenty-four measurements were made from 22 serial thin sections cut parallel to both the X-Z and X-Y planes of finite strain, giving an average extension of 1.72 parallel to the X-direction of finite strain. The least-squares best-fit line to a plot of initial length vs change in length gives an R2 value of 0.998. A minimum estimate of maximum shortening (negative e) was also made by measuring the total lengths of S1 folia that had been crenulated during OCM formation, giving a value of −0.54, which falls short of the −0.63 expected for constant-volume, plane-strain deformation. Because the OCM method is particularly suited to metapelites, results may provide new insight into mechanisms of folding and crenulation cleavage development, pressure-temperature-time-deformation histories, mass transport during deformation and metamorphism, and kinematic studies of porphyroblast behavior (rotation vs non-rotation) during ductile deformation.