diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3500017f..ac2ed6ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,8 @@
 Unreleased ([changes](https://github.com/colszowka/simplecov/compare/v0.7.0...master))
 -------------------
 
+  * [FEATURE] Automatically detect the usage of parallel_tests.
+    See https://github.com/colszowka/simplecov/issues/64
   * [FEATURE] Adds support for Rails 4 command guessing.
 
 v0.7.1, 2012-10-12 ([changes](https://github.com/colszowka/simplecov/compare/v0.7.0...v0.7.1))
diff --git a/lib/simplecov.rb b/lib/simplecov.rb
index 0db017fe..50739b2d 100644
--- a/lib/simplecov.rb
+++ b/lib/simplecov.rb
@@ -1,6 +1,8 @@
 #
 # Code coverage for ruby 1.9. Please check out README for a full introduction.
 #
+require 'lockfile'
+
 module SimpleCov
   # Indicates invalid coverage data
   class CoverageDataError < StandardError; end;
@@ -52,8 +54,12 @@ def result
       # If we're using merging of results, store the current result
       # first, then merge the results and return those
       if use_merging
-        SimpleCov::ResultMerger.store_result(@result) if @result
-        return SimpleCov::ResultMerger.merged_result
+        lockfile = ::Lockfile.new ResultMerger.resultset_path + '.lockfile'
+
+        lockfile.lock do
+          SimpleCov::ResultMerger.store_result(@result) if @result
+          return SimpleCov::ResultMerger.merged_result
+        end
       else
         return @result if defined? @result
       end
diff --git a/lib/simplecov/command_guesser.rb b/lib/simplecov/command_guesser.rb
index 4fb8b1fb..c27984a0 100644
--- a/lib/simplecov/command_guesser.rb
+++ b/lib/simplecov/command_guesser.rb
@@ -11,10 +11,19 @@ class << self
     attr_accessor :original_run_command
     
     def guess
-      from_command_line_options || from_defined_constants
+      from_env || from_command_line_options || from_defined_constants
     end
     
     private
+
+    def from_env
+      # If being run from inside parallel_tests set the command name according to the process number
+      if ENV['PARALLEL_TEST_GROUPS'] && ENV['TEST_ENV_NUMBER']
+        number = ENV['TEST_ENV_NUMBER']
+        number = '1' if number == ''
+        "(#{number}/#{ENV['PARALLEL_TEST_GROUPS']})"
+      end
+    end
     
     def from_command_line_options
       case original_run_command
diff --git a/simplecov.gemspec b/simplecov.gemspec
index 65eae894..fa1fb229 100644
--- a/simplecov.gemspec
+++ b/simplecov.gemspec
@@ -13,6 +13,7 @@ Gem::Specification.new do |gem|
   gem.summary     = gem.description
 
   gem.add_dependency 'multi_json', '~> 1.0'
+  gem.add_dependency 'lockfile'
   gem.add_dependency 'simplecov-html', '~> 0.7.1'
 
   gem.add_development_dependency 'aruba'