We report the serendipitous discovery of a supernova ( SN ) in the nearby radio-galaxy 3C 78 .
Observations obtained with the STIS spectrograph on board the Hubble Space Telescope show , at a distance of 0.54 arcsec ( 300 pc ) from the galaxy nucleus , a second bright source , not present in previous images .
As this source was fortuitously covered by the spectrograph slit its spectrum was obtained and it is characteristic of a Type Ia SN .
This SN is closely aligned with the radio-jet of 3C 78 .
Analysis of historical records shows that such a close association between jet and supernova occurred in 6 of the 14 reported SNe in radio-galaxies .
The probability that this results from a random distribution of SN in the host galaxy is less than \sim 0.05 % .
We then argue that jets might trigger supernova explosions .