To cite zoo in publications use:

Achim Zeileis and Gabor Grothendieck (2005). zoo: S3 Infrastructure for Regular and Irregular Time Series. Journal of Statistical Software, 14(6), 1-27. URL http://www.jstatsoft.org/v14/i06/

Corresponding BibTeX entry:

  @Article{,
    title = {zoo: S3 Infrastructure for Regular and Irregular Time
      Series},
    author = {Achim Zeileis and Gabor Grothendieck},
    journal = {Journal of Statistical Software},
    year = {2005},
    volume = {14},
    number = {6},
    pages = {1--27},
    url = {http://www.jstatsoft.org/v14/i06/},
  }