After giant HII regions, which have been used for decades to determine metallicities and metallicity gradients in external galaxies, it is the turn of planetary nebulae - which are much fainter but can now be observed with large telescopes. The comparison of HII regions and planetary nebulae in the same galaxy leads to several surprises: we will present the results of our recent work on the galaxy NGC 3109, a galaxy of Magellanic type in the Local Group.