BLUE compact dwarf galaxies are noted for their high star-formation activity and young age. They can therefore be used to verify models of galaxy formation, the chemical evolution of matter and the evolution of massive stars, for example. Here we report observations of the blue compact dwarf galaxy, SBS0335–052, which show that this galaxy has an extremely low heavy-element abundance; the oxygen abundance is 77-times lower than the solar value and 1.7-times lower than that found in another galaxy, I Zw 18, which was the most deficient in heavy elements. The electron temperature, 24,800 K, is very high which leads us to conclude that there are a significant number of stars with masses≈100 M⊙ and effective temperatures of ionizing stars of up to 8 x 10⁴K in the galaxy. Our observations imply that SBS0335–052 is very young.