The paper presents simultaneous monitoring of the BL Lacertae object Markarian 501 obtained in the visible and near infrared using a double photometer system on the 1.5 m Carlos Sanchez Telescope situated in El Observatorio del Teide (Tenerife) and CCD photometry in U and B taken with the 1 m Jacobus Kapteyn Telescope of El Observatorio del Roque de los Muchachos (La Palma). Evidence is seen of coordinated variability on time scales of hours in the variations in the visible and infrared. A brief infrared flare is seen on one night which has a formal probability of reality of approximately 99 percent and which is not seen in the B light curve. If real, this flare presents considerable difficulties with the framework of existing models. The results demonstrate the value of having a single telescope equipped to observe simultaneously in two different ranges of the spectrum. These data are compared with CCD photometry in U and B taken at other epochs.