Cooperating robots have the potential to achieve tasks more quickly and with a higher probability of success than a single robot acting alone. This paper describes a system of miniature mobile robots and the algorithms used to demonstrate cooperative plume tracking and source localization. The robots share sensor information, but individually decide a course of action based on their estimate of the plume field. This algorithm was implemented on a group of miniature mobile robots capable of measuring temperature plumes. Experimental results are presented for a representative test run. We also describe a 'next-generation' miniature mobile robot under development that occupies only 0.25 cubic inches in volume.