Abstract: A light bulb tester includes a body having first and second opposite ends. A pair of parallel cylindrical electrically-conductive wire probes on the first end are spaced apart a distance of from about 4.5 mm to about 6.0 mm, said distance including the widths of the two probes and the length of the gap formed between them. A pair of male and female nine-volt battery contacts are attached on the second end of the body so that the tester may be powered by a standard battery of the 9V (household transistor) size. The tester is used for testing a bulb of a holiday light bulb string of the standard type having a plurality of bulbs in sockets wired in series. The bottom of each socket forms an aligned pair of apertures or a slot leading to a pair of electrical socket contacts inside the socket. There is enough room in the apertures (or slot) to insert the probes and to connect them to the socket contacts in order to test the bulb while it remains in the socket.