The realtive penetration depths of a series of In-Sn, In-Tl, and In-Pb alloy specimens were measured at 700 kc/sec with and without a longitudinally applied dc magnetic field. In the dilute range the zero-field penetration depth showed a mean free path dependence roughly proportional to the one-half power of the resistivity, in accordance with previous work, but for samples with resistivities exceeding approximately 1.3 μΩ cm the effective penetration depth became anomalously large, indicating filamentary behavior. Measurements of the magnetic field dependence failed to show the decrease in penetration depth with applied field found by experimenters working at microwave frequencies; instead there was reasonable agreement with the predictions of the Ginzburg-Landau theory. The experimentally estimated values of the parameter κ at which filamentary characteristics appeared were of the order of 0.8±0.05 as compared to the theoretically derived critical value of 12. © 1963 The American Physical Society.