Three thousand kilometers from tidewater, a porpoise surfaces. Then a foot-long skinny slasher of a fish grabs the streamer fly you strip through water the color of Lapsong tea and shreds it. Sweat runs down your nose. The February sun blazes.

Welcome to the Amazon, and to a tropical watershed of three million square miles that empties an astonishing equivalent of seven Mississippi Rivers into the Atlantic. Welcome to a misty-morning world of chattering green parrots and to an aquatic Eden wherein swim 2,500 species of freshwater fish.

But enough geology and hydrology and biology. Let's go fishing in Brazil's Rio Negro, the Amazon's largest left tributary and the world's largest blackwater river. Be prepared to cast nonstop. Cast until it hurts, then strip your fly as fast as you've ever stripped it back to a beat-up tin bass boat.

The guys and gals throwing treble-hooked plugs the size of hatchery rainbows are outfishing you. Get used to it. With rods as stiff as pool cues and rapid-retrieve casting reels, they make several casts half the length of a football field to every one you struggle with your 10-weight to get a big, air-resistant popper out around first-down territory. Ah-WHOOSH! That's the sound of these huge surface lures spraying water every which way. Ka-POOSHHH! That's the sound of a 20-pound tucunaré açu peacock bass crushing the lure with the finesse of a Labrador retriever hitting a swimming pool.

You catch many exquisitely colored yellow tucunaré borboleta-butterfly peacock-with radiant blue fins and scarlet eyes, weighing one to two pounds. Then one evening your guide dips his hand-carved wooden paddle into a mirror-still inky lagoon and points to a patch of surface movement as subtle as pinpricks. "Criancinha," he whispers-babies. You drop your garish streamer on the barely discernable riffle and strip long and slowly. A swirl the size of a washtub engulfs your fly. The calm surface explodes in fury. Your rod plunges violently. Overhead against the red sky, two dozen macaws-their wings and tails fanned in magnificent flight-are heading to roost in the Brazilian nut trees.