World 5-1 is the first level of World 5 in Super Mario Bros. This level introduces the Bullet Bill and Bill Blaster enemies into the series.
The player starts in front of a row of Koopa Troopas and Goombas, ending at pipes with Piranha Plants and a gap between the two. After the second pipe is a row of Goombas and Koopa Paratroopas, after which is a block structure with a row of Brick Blocks above it, the middle of which containing a Super Star. Afterward are more Goombas and Koopa Troopas along with the new Bill Blaster. Beyond the row of enemies is a structure of blocks, and the gap between the Brick Blocks and the stone pillar is a Hidden Block containing a 1-Up Mushroom. The player can also use the blocks as a platform to reach the Warp Pipe above, which leads to an underground area with coins, and the exit pipe will take them back above ground near where they left. Around the pipes are two Bill Blasters and beyond those are two Koopa Paratroopas jumping down the stone stairs and floating pillar before the Goal Pole.