A subcritical cascade R-744 refrigeration system including a defrost system having at least one high stage compressor for compressing a first refrigerant from a low pressure first refrigerant vapor into a high pressure compressed first refrigerant vapor, a high stage condenser operatively connected to the high stage compressor for condensing the compressed first refrigerant vapor at least partially into a condensed first refrigerant, a high stage heat exchanger operatively connected to the high stage condenser for receiving the condensed first refrigerant, at least one low stage compressor, operatively connected to the high stage heat exchanger, for compressing a second refrigerant comprising R-744 refrigerant from a low pressure second refrigerant vapor into a high pressure compressed second refrigerant vapor, the compressed second refrigerant vapor being condensed into condensed second refrigerant liquid having a refrigerant temperature between -7 and -4 degrees Centigrade in the high stage heat exchanger by absorption of heat therefrom by the condensed first refrigerant which is evaporated into the low pressure first refrigerant vapor for circulation to the high stage compressor, connected to the high stage heat exchanger, for subsequent compressing, the second refrigerant liquid being transmitted to a reservoir connected to the high stage heat exchanger for storage and from the reservoir, during a refrigeration cycle to at least one evaporator of a plurality of evaporators for absorbing load heat from a thermal load to cool the thermal load and being at least partially evaporated by the load heat into the low pressure second refrigerant vapor for subsequent compressing by the low stage compressor, the defrost system being characterized by: - a defrost inlet heat exchanger operatively connected to the high stage condenser, the high stage heat exchanger, and the reservoir; and - at least one defrost compressor operatively connected to each evaporator and to said defrost inlet heat exchanger for circulation therethrough of the condensed first refrigerant from the high stage condenser to the high stage heat exchanger and a defrost portion of the second refrigerant liquid between the reservoir and the defrost compressor during the defrost cycle, the defrost portion being evaporated in said defrost inlet heat exchanger by absorption of an additional defrost heat load from the condensed first refrigerant and circulated to said defrost compressor for compression thereby during the defrost cycle into a high pressure defrost vapor portion, said defrost vapor portion being circulated from said defrost compressor into a selected evaporator of the evaporator, frost on said selected evaporator being melted by absorption of a defrost heat from said defrost portion, thereby defrosting said selected evaporator and at least partially condensing said defrost vapor portion into a condensed defrost portion, said condensed defrost portion being circulated from said selected evaporator to the high stage heat exchanger for condensation therein into the second refrigerant liquid and circulation to the reservoir, said additional defrost heat load ensuring that said defrost heat for said defrost portion is sufficient to melt said frost. - a defrost inlet heat exchanger operatively connected to the high stage condenser, the high stage heat exchanger, and the reservoir; and - at least one defrost compressor operatively connected to each evaporator and to said defrost inlet heat exchanger for circulation therethrough of the condensed first refrigerant from the high stage condenser to the high stage heat exchanger and a defrost portion of the second refrigerant liquid between the reservoir and the defrost compressor during the defrost cycle, the defrost portion being evaporated in said defrost inlet heat exchanger by absorption of an additional defrost heat load from the condensed first refrigerant and circulated to said defrost compressor for compression thereby during the defrost cycle into a high pressure defrost vapor portion, said defrost vapor portion being circulated from said defrost compressor into a selected evaporator of the evaporator, frost on said selected evaporator being melted by absorption of a defrost heat from said defrost portion, thereby defrosting said selected evaporator and at least partially condensing said defrost vapor portion into a condensed defrost portion, said condensed defrost portion being circulated from said selected evaporator to the high stage heat exchanger for condensation therein into the second refrigerant liquid and circulation to the reservoir, said additional defrost heat load ensuring that said defrost heat for said defrost portion is sufficient to melt said frost.