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We investigate the decays D{sup 0}{yields}{pi}{sup -}l{sup +}{nu} and D{sup 0}{yields}K{sup -}l{sup +}{nu}, where l is e or {mu}, using approximately 7 fb{sup -1} of data collected with the CLEO III detector. We find R{sub 0}{identical_to}B(D{sup 0}{yields}{pi}{sup -}e{sup +}{nu})/B(D{sup 0}{yields}K{sup -}e{sup +}{nu})=0.082{+-}0.006{+-}0.005. Fits to the kinematic distributions of the data provide parameters describing the form factor of each mode. Combining the form factor results and R{sub 0} gives vertical bar f{sub +}{sup {pi}}(0) vertical bar {sup 2} vertical bar V{sub cd} vertical bar {sup 2}/ vertical bar f{sub +}{sup K}(0) vertical bar{sup 2} vertical bar V{sub cs} vertical bar{sup 2}=0.038{sub -0.007-0.003}{sup +0.006+0.005}.