This is the forth paper from a series of articles on governing of different types of transactions in Bulgarian farming applying the framework of New Institutional and Transaction Cost Economics. It is based on a large scale microeconomic data from 194 typical commercial farms of different sizes (small, middle-size, big) and types (individual, family and group farms; cooperatives, agrofirms) from all regions of the country. This study concentrate on factors and modes for organization of service supply in Bulgarian farms. Different governing forms (internal production, own cooperative, member-organization, group supply, market contragent) for major service supply (technological knowledge and advises, mechanization services, maintenance of machinery and equipment, chemicals and pesticides dispersion, veterinary service, others) in farms of different types and sizes has been determined. Microeconomic factors responsible for blocking of a particular service supply, and for choosing of various organizational and contract modes for service supply have been specified. Dominant governing modes have been explained by comparative advantages for saving on transacting costs (for finding partners and needed service, contracting, contract enforcement and disputing, renewal of contracts etc). Extension (optimization) of farm size through some mode of service supply is undertaken only if it is less expensive comparing to other forms for farm enlargement (e.g. land supply, labor supply, material inputs supply, vertical integration etc). Transaction costs economizing framework has been used through analysis for each type of services of: reasons for non using service supply or for own execution of such activities (no need, having own necessary qualification, having a specialist at farm, lack of a supplier, quality problem, contracting problem, contract execution problem, high service price); reasons for choosing of outside suppliers (lack of another supplier in the region, good price, high quality, lack of problems in contracting ...
