We have used high-Tc SQUIDs to study the phase transition in a film of Mn3O4. The film was positioned on top of the SQUID, and the temperature of the film and the SQUID was varied in the range 30-60 K. By monitoring the voltage at the SQUID output we were able to observe a reproducable shift in the SQUID response in the range 30-50 K. This shift is related to the transition from the paramagnetic to the ferrimagnetic state of the Mn3O4 film.