- Ice Cream & Frozen Yogurt, Janesville
- Ice Rinks, Janesville
- Ice Sports Clubs & Instruction, Janesville
- Ignition Apparatus Manufacturers, Janesville
- Individual & Family Information & Referral Services, Janesville
- Individual & Family Services, Janesville
- Industrial & Commercial Cleaning Services & Companies, Janesville
- Industrial & Commercial Real Estate, Janesville
- Industrial & Heavy Equipment Repair & Service, Janesville
- Industrial & Heavy Equipment Vehicle Rental & Leasing, Janesville
- Industrial & Heavy Vehicle Dealers, Janesville
- Industrial Contractors, Janesville
- Industrial Engineers & Designers, Janesville - Architectural Designers
- Industrial Equipment & Machinery, Janesville
- Industrial Equipment & Systems, Janesville
- Industrial Filters, Janesville
- Industrial Goods & Products - Hard Goods - Large, Janesville
- Industrial Goods & Products - Hard Goods - Small, Janesville
- Industrial Goods & Products - Soft - Large & Small, Janesville
- Industrial Heaters & Boilers, Janesville
- Industrial Lifts, Janesville
- Industrial Machinery Supplies & Equipment - Leasing & Rental, Janesville
- Industrial Manufacturers, Janesville
- Industrial Pump Repair & Service, Janesville - Centrifugal, Positive Displacement, Submersible, Diaphragm, Hydraulic, Water, Vacuum Pumps, etc
- Industrial Pumps, Janesville - Centrifugal, Positive Displacement, Submersible, Diaphragm, Hydraulic, Water, Vacuum Pump, etc
- Industrial Supply, Parts & Equipment - Miscellaneous, Janesville
- Industrial Trucks, Janesville
- Industry, Janesville
- Infectious Disease Specialists, Janesville - Doctors, Physicians & Surgeons
- Information Bureaus & Services, Janesville
- Information Retrieval Services, Janesville
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- Information Technology Services, Janesville - IT Service
- Instruments & Control Equipment, Janesville
- Insulation & Energy Conservation Materials, Janesville
- Insulation Contractors, Janesville - Blown-In, Spray Foam, Batt, Fiberglass, Attic Insulation Installation & Service
- Insulation Material , Janesville - Contractors Supplies & Equipment
- Insurance Brokers, Janesville
- Insurance Carriers, Janesville
- Insurance Carriers By Specialty, Janesville
- Insurance Contractors, Janesville
- Insurance Farm, Janesville
- Insurance Flood, Janesville
- Insurance General Liability, Janesville
- Insurance Marine, Janesville
- Insurance Motorcycles, Janesville
- Insurance Renters, Janesville
- Insurance Services, Janesville
- Interior Designers, Janesville
- Interior Designers & Decorators, Janesville
- Internal Medicine Doctors, Janesville - Internists Physicians & Surgeons
- Internet Development, Janesville
- Internet Home Page Development & Consulting, Janesville
- Internet Marketing & Advertising Services, Janesville
- Internet Providers, Janesville
- Internet Services, Janesville
- Investment Bankers, Janesville
- Investment Management Services, Janesville
- Investment Services & Advisors, Janesville
- Iron, Steel & Metal Fabricators, Janesville - Fabrication Companies
- Irrigation Systems Repair & Service, Janesville
- Italian Restaurants, Janesville
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