The Agency did not exercise due diligence in its attempts to locate Father for service when it failed to follow up with unique facts more likely to yield his whereabouts. The minor and his younger sibling were removed from Mother and a petition was filed. Mother reported that Father was in jail and had an upcoming hearing date but could not provide Father’s exact date of birth. Mother also reported that paternal grandfather lived on her street. The Agency reported that Father’s whereabouts were unknown and filed a declaration of due diligence which stated that it had searched Internet law enforcement and prison databases and phone listings for Father’s name. The Agency did not use Father’s middle name or birth year to narrow the search. After an unsuccessful attempt at reunification with Mother, a section 366.26 hearing was set. The juvenile court granted the Agency’s application to provide notice to Father by way of publication. No notice was provided to paternal grandparents. At the section 366.26 hearing, the juvenile court terminated parental rights. Mother appealed and the Court of Appeal reversed. Due process requires child welfare agencies to exercise reasonable diligence in attempting to locate and notify parents of dependency proceedings. This requires a thorough and systematic investigation which includes not only the standard avenues available to help locate a missing parent but also the specific ones most likely under the unique facts known to the Agency to yield a parent’s whereabouts. Here, the Agency neglected to utilize information such as Father’s middle name and birth year to assist in their search. Additionally, the Agency had a specific court date for Father, which they did not use to try and narrow their search. More significantly, paternal grandfather lived down the street from Mother, but the Agency did not follow up with this more specific avenue of information. This was not a reasonably diligent effort. The fact that Father was served notice by publication does not remedy the inadequate search. Service by publication will not satisfy due process if it is not the most likely means of notifying the parent. The error was not harmless because there is no evidence to suggest Father would have nonetheless failed to assert his rights had he been given proper notice. Because the matter must be remanded to address the notice issues to Father, the Agency must also complete its duty of initial ICWA inquiry.