Yes, a PAR 16 bulb will fit in a MR16 GU10 fixture because they both have 2 inch widths. The lengths of a PAR16 bulb and a MR16 GU10 fixture, however, are different. The MR16 GU10 fixture 2.10 inches long whereas a PAR16 lamp is 3.15 inches long. In short, a PAR16 lamp will fit in a MR16 GU10 fixture but part of the bulb will be exposed. Whether you can tolerate this will depend on your preference.