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The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron pp̅ collider. Using a 0.9 fb-1 dataset, we apply a multivariate analysis to separate signal from background and measure σ (pp̅ → tb + X, tqb + X) = 4.9 ± 1.4 pb. The probability to measure a cross section at this value or higher in the absence of a signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the Cabibbo- Kobayashi-Maskawa matrix element that describes the Wtb coupling and find 0.68 < |V tb| ≤ 1 at 95% C.L. within the standard model.