An electrical connector assembly includes male and female connector bodies formed with a plurality of terminal cavities in which electrical cables are disposed with the terminals secured against removal from the cavities by flexible lances engaging walls within the terminal cavities. A seal on the cables closes and seals the cavities in which they are installed. At least one of the cavities is unoccupied by a cable and is closed by a non-electrically conductive locking cavity plug. The locking cavity plug has a flexible lance that engages a wall within the unoccupied cavity in which it is installed to secure the plug against removal and carries a seal which engages and seals the unoccupied cavity.