PNG135.9+55.9 is a halo planetary nebula recently discovered among the objects from the Second Byurakan Survey. Based upon our first observations, a photoionization model analysis shows that the oxygen abundance in the nebula must be extremely low, probably 10-2–10-3of the solar value. This finding has strong implications on our understanding of the formation of planetary nebulae, of mixing processes in the stellar progenitors and of the evolution of the Galactic halo.