scala> @serializable class Foo
scala> val f = new Foo
scala> f.isInstanceOf[java.io.Serializable]
res0: Boolean = true

object Test {
  @serializable class Hoe {
    def add(a: java.io.Serializable): Unit = println(a)
    def someMethod() { add(this) }
  } 
}

[emberson@olympia ser2]$$ make
Test.scala:5: error: type mismatch;
 found   : Test.Hoe
 required: java.io.Serializable
    def someMethod() { add(this) }

  def someMethod() { add(this.asInstanceOf[java.io.Serializable]) }