I'm currently playing a world I created in august 2012 (shortly after the release of 1.3.2 update) and I built today my first Nether portal in this world, as I was waiting for the redstone update to go in the Nether.
But as I entered the Nether I could'nt hear any mob screams, neither ghasts nor zombie pigmen. I can't tell about wither skeletons or blazes as there is no fortress next to the portal and I haven't gone far in the Nether as for now.
But anyway I should have seen at least some ghasts floating in the "sky" and heard zombie pigmen, so why are there none of them ?
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Yeah you're right, I was on peaceful ^^;
Cannot reproduce.
please make sure you are not on peaceful.
Attached screenshot of a ghast and a magma cube.