Deputies said the inmate was found unresponsive Saturday night during routine cell checks. Ventura County fire personnel and paramedics were called, but the inmate was pronounced dead at the jail.
Officials said the death appears to be from natural causes and that there was no indication of violence or suspicious circumstances.
An autopsy will be performed to try to determine the cause of death. The name of the inmate was not immediately released, pending notification of next of kin.