I will however, summarize the performance of each unit and describe their prospects for 2012 and beyond. Houston Electric had its best year financially. Core operating income was $496 million compared to $427 million in 2010. Fourth quarter income was $62 million, up $6 million from 2010. Last summer's record heat was the biggest driver of Houston Electric's increased income.
In addition, we saw strong growth in Houston with over 45,000 new customers added last year. Our gas LDCs also had a good year. Operating income for the full year was $226 million or about $5 million below the record level of 2010. Fourth quarter income was $73 million compared to $86 million in 2010. While margin growth was modest last year, we were successful in our expense management efforts. As a result, on an overall basis, we earned at or near our authorized rate of return for the second consecutive year.
Our Interstate Pipelines achieved operating income of $248 million last year down from the $270 million in 2010. Income in the fourth quarter was $52 million compared to $63 million in the previous year. The decline in earnings for the full year and fourth quarter were almost completely attributable to the exploration of a backhaul contract on our Carthage to Perryville line.
Our Field Services unit had a very strong year. As a result of the investments we've made to gather and treat gas in several developing shale plays. Full year operating income was $189 million, compared to $151 million the previous year. Fourth quarter earnings were $53 million compared to $57 million in the fourth quarter of 2010. It is worth noting that operating income for 2010 reflected a gain of $21 million from the sale of some nonstrategic assets. Excluding that onetime gain, last year's fourth quarter earnings were up over 45% from 2010.Read the rest of this transcript for free on seekingalpha.com