- Laboratories Medical, Southold
- Land Surveyor, Southold - Land Surveying Services
- Land Use & Zoning Attorneys & Lawyers, Southold
- Landscape Contractors & Designers, Southold
- Landscape Garden Services, Southold
- Landscaping, Southold
- Landscaping Materials & Supplies, Southold
- Lawn & Garden Equipment & Supplies, Southold
- Lawn & Garden Services, Southold
- Lawn & Garden Sprinkler Systems, Southold
- Lawn Maintenance Services, Southold
- Lawn Mower Repair & Blade Sharpening Service, Southold
- Lawn Spraying & Treatment, Southold
- Lawyers & Attorneys, Southold
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- Legal Professionals, Southold
- Libraries, Southold
- Licensed Counselors, Southold
- Limousine Service, Southold
- Liquor Stores, Southold
- Loans Personal, Southold
- Locks, Southold
- Locksmiths, Southold - Residence, Commercial, Safe & Auto Car Locksmith
- Locksmiths Equipment & Supplies, Southold
- Lodging, Southold
- Long Haul Trucking Companies, Southold
- Lp Gas Equipment & Supplies, Southold
- Lumber & Lumber Products, Southold
- Lumber & Lumber Products, Southold
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