Osutchi and Mesutchi!
The original mating Tamagotchi!
All the text is Japanese but is very easy to figure out if you are at all familiar with the regular Tamagotchis at all.
The first color assortments were released in December of 1997

Osutchi and Mesutchi mate.
When they do, the babies stay on your screen while the parents leave after a couple of days.
I have only got as far as three generations though they can go several generations and you can get a LOT of characters.
This page was last updated on: November 3, 2007
It took a long time, but I now have all of the osu mesu pairs!
Growth chart