We report on a search for second generation leptoquarks (LQ2) which decay into a muon plus quark in collisions at a center-of-mass energy of in the D0 detector using an integrated luminosity of about 300 pb−1. No evidence for a leptoquark signal is observed and an upper bound on the product of the cross section for single leptoquark production times branching fraction β into a quark and a muon was determined for second generation scalar leptoquarks as a function of the leptoquark mass. This result has been combined with a previously published D0 search for leptoquark pair production to obtain leptoquark mass limits as a function of the leptoquark–muon–quark coupling, λ. Assuming λ=1, lower limits on the mass of a second generation scalar leptoquark coupling to a u quark and a muon are mLQ2>274 GeV and mLQ2>226 GeV for β=1 and β=1/2, respectively.