There are three types of pests: Shell Creepers, Side Steppers, and Fighter Flies.

* Shell Creepers: (possible inspiration for the later Koopa Troopas) look like turtles. They move slowly, though it will move the same speed as Mario or Luigi when it is hit three times and not kicked and it only takes one hit to flip them over. Later replaced with Spinys in the Super Mario Advance series.
* Side Steppers (later renamed Crabs): resemble monkeys and are often referred to as monkeys in small gaming communities. They move the fastest of all the pests, and it takes two hits to flip them over. After taking one hit, Side Steppers become angry and begin to move even faster than Mario and Luigi.
* Fighter Flies: They move by taking short hops, and can only be hit when touching the ground.

There are other enemies and obstacles that do not need to be killed to clear the phase.

* Slipice (later renamed Freezie): They move slowly. A hit instantly kills them. A new one is respawned in a few seconds if the previous one is killed. They can freeze a platform, turning it into ice, making it harder to maneuver Mario. If a platform is iced, it is returned to normal at the start of the next phase. If all three ice-eligible platforms are iced, no more slipices spawn for that phase.
* Icicles: They start appearing on Phase 16, after the third coin phase. They will fall down to cause harm but may be killed when they're only a drop attached to the roof.
* Red fireballs: They bounce diagonally. Mario can kill a fireball by hitting the ground as the fireball bounces; killing a fireball is unwise, as one soon respawns and moves faster. If the red fireball makes a complete circuit of the screen, it disappears and then respawns moving faster, effectively limiting the amount of time Mario can safely take to complete a phase. Normally, only one fireball is present; after the fifth coin phase, Phase 29, two fireballs are present simultaneously.
* Green fireballs (sometimes called Hurricanes): They move horizontally, spawning on one end of the screen and travelling to the other. They bounce at irregular intervals, making it risky to jump over one.



Phases

* Phase 1-2. Shell Creepers
* Phase 3. First coin phase; POW not replenished
* Phase 4. Side Steppers
* Phase 5. Side Steppers and Shell Creepers; this is the only phase in the game that has both Side Steppers and Shell Creepers.
* Phase 6. Fighter Flies
* Phase 7. Fighter Flies and Side Steppers
* Phase 8. Second coin phase; POW replenished
* Phase 9. Slipice are now present on every phase; Fighter Flies and Shell Creepers.
* Phase 10, 11. Fighter Flies and Side Steppers
* Phase 12-14. Same as 9-11, but the Fireballs are more frequent
* Phase 15. Third coin phase; floors turn invisible; POW replenished
* Phase 16-21. Icicles are now present on every phase; otherwise same as 9-14. Fireball frequency is decreased on phases 16-18.
* Phase 22. Fourth coin phase; POW replenished
* Phase 23-28. Same as 16-21, but fireball frequency is maximum and Icicles are more common.
* Phase 25. Phase counter replaced with a red "KO".
* Phase 29. Fifth coin phase; POW replenished
* Phase 30-35. Same as 23-28, but now two red fireballs appear on each phase.
* The same pattern of six phases repeats for the remainder of the game.