Abstract <p>The bioconcentration factors and the associated forms of occurrence of neptunium and plutonium in soil and bottom sediment samples collected across a wide interregional geographic range of the Russian Federation were examined. Neptunium was determined using a luminescence method on activated crystal phosphors, with a detection limit of 1.8 × 10^–4 Bq, or 5 × 10^–12 g. Plutonium was determined by alpha spectrometry (10^–3 Bq). The study demonstrates that the specific properties of neptunium lead to higher relative bioconcentration of this element, reaching absolute activity levels comparable to those of plutonium. It should be noted that long-lived ²³⁷Np is becoming one of the main dose-producing radionuclides in radioactive waste over time. Thus, radioecological study of the behavior of neptunium is of particular relevance and requires further development.</p>