Route 11
Pokemon Found: Spearow, Ekans (Red), Nidorina, Drowzee, Sandshrew (Blue)
After defeating Lt. Surge, the path to the east will open and you will be able to procede to Route 11. As you wander on the sidewalks, you will be challenged by no fewer than ten trainers, each with up to four Lv. 20 Pokemon. The wild Pokemon found here are not very strong, but they possess the power to make your monsters fall asleep! When you enter the toll gate, you'll be warned that the road is blocked ahead by a sleeping Snorlax! As long as you're here, you may as well go up into the observing tower. If you have 30 Pokemon, you will get an Itemfinder from Prof. Oak's aide!

Diglett's Cave
Pokemon Found: Diglett, Dugtrio
Since Route 12 is blocked off, you will have to take a detour to Lavender Town. Return to the entrance to Route 11 and enter the cave opening. This is also the only place you will find Diglett and Dugtrio. They aren't terribly powerful, but there's a lot of them. You will emerge back on Route 2, on the other side of Virmilion Forest, where you couldn't go before. Go south to collect the items that you couldn't reach before. Inside the observing tower, Prof. Oak's aide awaits to give you the FLASH HM... as long as you have 15 Pokemon! You'll need it to enter Rock Tunnel.