The information reads as follows:
"Samus' adventure on Nintendo GameCube actually takes place in the past. After the events of the first Metroid adventure but before Metroid II, Samus learns that there were space pirate vessels in orbit as she fought on the surface of Zebes.

She traces one of these to an orbit around a planet called Tallon IV and discovers an enormous space pirate research operation in process. The space pirates are experimenting with a dangerous mutagen called Phazon, combining it with local lifeforms as they try to rebuild their army.

Samus must delve deep into magna-filled cavarns , poisonous mines and icy caves as she tries to stop the space pirates sinister plan."

Setting Prime between the first two Metroids is sure to please fans, filling them in on what happened in-between their favourite adventures. Also revealed was Metroid Fusions story.....

"Metroid fusion occurs shortly after Super Metroid. The absence of the Metroids has thrown the food chain out of balance on SR-388, and a parasite virus called X (which was the Metroids natural food) is multiplying. Samus is infected while researching it and the only way to save her is with the vaccine made from the DNA of the Metroid hatchling.

Because of the Metroid DNA in her body she can now absorb the X parasites to recover health and upgrades; a good thing as the X quickly take over a research station orbiting SR388. As Samus fights against the rising tide of parasites, she is also being hunted by the SA-X, a parasite mimicking her at the height of her powers."

So there you have it folks, Metroid Fusion takes place after Super Metroid, making it the latest in the series.

Source: Planet GameCube