The phenylseleninyl radical has been trapped in an X-irradiated single crystal of Ph₂SeO. The analysis of the hyperfine interaction with ⁷⁷Se shows that the unpaired electron is mainly localized (~80%) in a selenium 4p_x-orbital. The ESR parameters are quite similar to those obtained for the radical Ph₂As and indicate that the p_x-orbital is probably perpendicular to the CSeO plane. The spin density on the selenium atom in the phenylseleninyl radical is twice as great as that in the phenylselenonyl radical.